How can I make a program edit another program?

edit by Alex: Moved this topic to a more appropriate location, also adjusted the title to be more descriptive.

You can only do that from within an assembly program Welcome to Cemetech by the way!

I think I remember doing something like that with the old DCS7 libs on the TI-83+. Fun times.

Yes, there are functions to edit programs (and much more!) in the Celtic libraries which are included in the Doors SDK. The documentation for those functions is available on this page.

Editing programs from within programs can be done with either hybrid libs or just full on assembly. Both the monochrome and color versions of Doors offer the necessary libs to edit programs in a pretty intuitive way. I can't remember the exact command numbers off the top of my head, but those are documented in the SDKs)
As for assembly, you can directly change the bytes you want in memory to get what you want, but I would advise against this method because it's a lot more work even for someone who knows asm.