assuming the program is otherwise unprotected and will not try to prevent or detect it, write a loader which injects a dll into the target process's memory and patches bytes in the appropriate place to call a function in your dll that changes the string however you wish. there are lots of tutorials on code injection, here are some good ones:
Three Ways to Inject Your Code into Another Process
A More Complete DLL Injection Solution Using CreateRemoteThread
Code Injection - A Generic Approach for 32bit and 64bit Versions
InjLib - A library that implements remote code injection for all Windows versions