Wintermute

Hi there!
Actually I want to use a modified SMS/MMS Application to be able to send and recieve MMS in Japan. I've found a modified one at modoco (Thread is easy to find with keywords " Android MMS Softbank " ).
My problem now that I don't have root access to my phone and I don't have enough time to use a computer for doing that.
My Idea is to change the name of the Application into "S-Mail" or something like that and change the install directory to install it as second application which I can delete when I don't need it anymore. Anyway it would not overwrite system files. In theory.

Is it possible to do that?
I actually use computer a lot but I don't have any knowledge of programming.
Can anyone check if it's possible to do that with the MMS.apk for me?
I would be really thankful because that would mean I can use my Android again