Thanks to Max in this post, I made an horizontal menu. But now I'm trying to make an overlay menu, i I don't find how to do that... Let's see what i got first.

So, I have a class MapScreen which display my map:

public class MapScreen extends MenuScreen

Then, I have in the same file the MenuScreen class like this that allows to display the horizontal menu when I press the MENU key:

abstract class MenuScreen extends MainScreen {
boolean mMenuEnabled = false;
CyclicHFManager mMenuManager = null;

public MenuScreen() {
    mMenuManager = new CyclicHFManager();
    mMenuManager.setBorder(BorderFactory.createBevelBorder(new XYEdges(4,
            0, 0, 0), new XYEdges(Color.DARKBLUE, 0, 0, 0), new XYEdges(
            Color.WHITE, 0, 0, 0)));
    mMenuManager.setBackground(BackgroundFactory
            .createLinearGradientBackground(Color.DARKBLUE, Color.DARKBLUE,
                    Color.LIGHTBLUE, Color.LIGHTBLUE));


    for (int i = 0; i < 10; i++) {
        Bitmap nBitmap = new Bitmap(60, 60);
        Graphics g = new Graphics(nBitmap);
        g.setColor(Color.DARKBLUE);
        g.fillRect(0, 0, 60, 60);
        g.setColor(Color.WHITE);
        g.drawRect(0, 0, 60, 60);
        Font f = g.getFont().derive(Font.BOLD, 40);
        g.setFont(f);
        String text = String.valueOf(i);
        g.drawText(text, (60 - f.getAdvance(text)) >> 1, (60 - f
                .getHeight()) >> 1);

        Bitmap fBitmap = new Bitmap(60, 60);
        g = new Graphics(fBitmap);
        g.setColor(Color.DARKBLUE);
        g.fillRect(0, 0, 60, 60);
        g.setColor(Color.GOLD);
        g.drawRect(0, 0, 60, 60);
        g.setFont(f);
        g.drawText(text, (60 - f.getAdvance(text)) >> 1, (60 - f
                .getHeight()) >> 1);

        BitmapButtonField button = new BitmapButtonField(nBitmap, fBitmap);
        button.setCookie(String.valueOf(i));
        button.setPadding(new XYEdges(0, 18, 0, 18));

        button.setChangeListener(new FieldChangeListener() {
            public void fieldChanged(Field field, int context) {
                Dialog.inform("Button # " + (String) field.getCookie());
            }
        });

        mMenuManager.add(button);
    }
}

protected boolean keyDown(int keycode, int time) {
    if (Keypad.KEY_MENU == Keypad.key(keycode)) {
        if (mMenuManager.getManager() != null) {
            delete(mMenuManager);
            mMenuManager.mCyclicTurnedOn = false;
        } else {
            add(mMenuManager);
            mMenuManager.getField(2).setFocus();
            mMenuManager.mCyclicTurnedOn = true;
        }
        return true;
    } else {
        return super.keyDown(keycode, time);
    }
}}

And finally my menu manager:

public class CyclicHFManager extends HorizontalFieldManager {
int mFocusedFieldIndex = 0;
public boolean mCyclicTurnedOn = false;

public void focusChangeNotify(int arg0) {
    super.focusChangeNotify(arg0);
    if (mCyclicTurnedOn) {
        int focusedFieldIndexNew = getFieldWithFocusIndex();
        if (focusedFieldIndexNew != mFocusedFieldIndex) {
            if (focusedFieldIndexNew - mFocusedFieldIndex > 0)
                switchField(0, getFieldCount() - 1);
            else
                switchField(getFieldCount() - 1, 0);
        }
    } else {
        mFocusedFieldIndex = getFieldWithFocusIndex();
    }
}

private void switchField(int prevIndex, int newIndex) {
    Field field = getField(prevIndex);
    delete(field);
    insert(field, newIndex);
}}

So as it is like this, it is working: when I press the MENU key, the menu appears, i can navigate between buttons, and it disappear when I press again the same key. The only problem is my menu isn't overlaying my map, it pushes the content up. I tried with the menu manager like in your first response, resizing the content manager but it is the same result.

Max gave me the link http://stackoverflow.com/questions/1497073/blackberry-fields-layout-animation to do so, but I really don't know how to use it to make it work in my project...

Thank you for your help!

UPDATE

This works great, it's what I wanted. However, I still have a problem because I'm under 4.5. So first in the MenuHostManager constructor, I deleted the

USE_ALL_HEIGHT  

and change

setPositionChild(mMenuManager, 0, 
    Display.getHeight() - mMenuManager.getPreferredHeight());  

like this to have the menu at the bottom of the screen. It worked.

Then, instead of drawing my bitmaps, I did this:

Bitmap nBitmap = EncodedImage.getEncodedImageResource("menu" + 
    i + ".png").getBitmap();
BitmapButtonField button = new BitmapButtonField(nBitmap, nBitmap);

And it worked too (no rollover for now, later). So it is great!
I also overwrite the Paint method of my CyclicHFManager to have a background color, because I can't use the BorderFactory and BackgroundFactory... My menu bar has a color for now so it's ok.
Then, because of these 2 classes missing, in my BitmapButtonField I had to delete the 2 setBorder functions that change the borders. And now i have my buttons pretty big like normal buttons with borders...
How can I make the same effect as the setBorder functions under 4.5? (BTW, setBorder is not working under 4.5 too...).
Thank you!