I know, the general answer will be yes, but here is my situation.

I got a plotting function from one MATLAB's toolbox and modified its m-file to draw what I need. Eventually this function became a part of program I would like to distribute as an open source (or under other license). Can I do this? Well, may be it was not wise to create a function in such a way, but I didn't think about distribution at that time. The function still depends on other functions in this toolbox, so a potential user supposed to have a license for it.

Any thoughts, recommendations?

Have you ever modified MATLAB's m-files directly?