How to add code to dll?

[WyvernX]

Another way is to just load the dll, add a trampoline to your own code, and return. If its out of process, maybe just create a code section, add code you want, and start a remote thread to run it.

I guess a lot depends on what your ultimate goal is. Just to extend some functionality, or add new functions, etc. Does the old functions need to call the new code etc.