Quite simply, no.  Self modifying code is not possible without disassembly of the generated programs becasue you need to get the address of the instruction to modify.  This is a bad practice becasue future versions and new optimizations change the size and order of the code which means you lose compatibility with newer versions.  That doesn't even include the fact that every time you modify your code, you have to disassemble again to get the address since it changes with how many bytes come before the SMC.  And yet another thing, it doesn't actually save very much space nor speed especially for the effort you would have to go through to implement it.  This does require greater than average assembly knowledge to do.

EDIT: Oops! I misread your post.  If you just want a write back, you can do this with axe commands by copying the running program in ram (address _E9D93) into the program itself which you can get via the getcalc() command.  You can also get the size of the amount of data you need to copy with that command by reading the 2 byte number directly before the pointer.

Quigibo, is there any way to get the name of the program from w/in it? Because I may compile a program as DEMO but someone could rename it to DEMO4. Is there any way to check, or do I have to read OP1 at the beginning of the program?

Now that I think about it my idea wouldn't work for massive maps , because every change would be overwritten when I recompile my program. I don't think very well sometimes

That said it might still be useful to know for people who wants to put their game save data directly inside their program

I've been thinking of how to use real SMC, and change the executing code, but it would require some h4x.  I have thought of a few possible methods, and will post about them when I figure more out.