Replace system apps

Jun 9, 2013
I'd like to ask if it is possible to replace a system app from one rom to another.
I'm running WIUI rom on my Mi2S and I want to return to the weekly MIUI rom.
WIUI has a modded mms.apk with striped unicode, so we can sent sms with 160 characters. What I want to do is to replace the MIUI's mms.apk with WIUI's and be able to use it with no problems.
How can I do it?
I don't want to use any 3rd party app and I want to know if this mod is possible for future reference