i have this Proc

CREATE Proc  [dbo].Salse_Ditail 
    -- Add the parameters for the stored procedure here
    @Report_Form  varchar(1)  ,
    @DateFrom datetime  ,
    @DateTo  datetime   ,
    @COMPANYID varchar(3),
    @All varchar(1) ,
    @All1 varchar(1)    , 
    @All2 varchar(1)   , 
    @All3 varchar(1)   ,
    @All4 varchar(1)  , 
    @All5 varchar(1)   ,  
    @Sector varchar(10),
    @Report_Parameter nvarchar(max)  
as
BEGIN
    -- SET NOCOUNT ON added to prevent extra result sets from
    -- interfering with SELECT statements.
    DECLARE @STRWhere nvarchar(max)

    IF @All5=0 AND @All4=0 AND @All3=0 AND @All2=0 AND @All1=0 and @All=1
        set @STRWhere=  N'and  Sector_id = @Sector' 

    if @Report_Form =1 or @Report_Form =3 or @Report_Form =4 

    SELECT   RETURNREASONCODEID, SITE,SITE_NAME,Factory_id,Factory_Name,Sector_id,sector_name,Customer_name,
         Customer_id,ITEMID,ITEMNAME,SALESMANID,SALESMAN_NAME,Net_Qty,Net_Salse,Gross_Sales,Gross_Qty,
         NETWEIGHT_Gross,NETWEIGHT_salse_Gross,NETWEIGHT_NET,NETWEIGHT_salse_NET,Return_Sales,Free_Good,
         CollectionAmount  

    FROM hal_bas_new_rep 

    WHERE   DATAAREAID =@COMPANYID AND INVOICEDATE >= @DateFrom 
        AND INVOICEDATE <= @DateTo and Report_Activti = @Report_Form 

    if  @Report_Form =2  

        SELECT   RETURNREASONCODEID ,  RETURNREASONDESC,  SITE , SITE_NAME , Factory_id ,
            Factory_Name ,  Sector_id ,  sector_name ,  Customer_name ,  Customer_id , 
            ITEMID , ITEMNAME , SALESMANID , SALESMAN_NAME ,    Return_Sales   

        FROM dbo.hal_bas_new_rep 

        WHERE  DATAAREAID =@COMPANYID AND INVOICEDATE >= @DateFrom 
            AND INVOICEDATE <= @DateTo and Report_Activti =   @Report_Form 
            and  RETURNREASONCODEID in
            (
                SELECT     Val 
                FROM dbo.fn_String_To_Table(@Report_Parameter,',',1)
            )  
            /*
            @STRWhere   question how i can but the virble here  
            */
end
GO

i want to but virble put a variable under where Expression and from this function buc I have many function i want to add

if any one have answer pls send me