Can you advise why I need this app as I have ringtones of mp3 file using on other phone and I just transfer the file to Windows Phone only.
I have no idea if there is an app to do this on your phone, but there are several free ways to do it on the desktop. I use a program (free!!) called MP3Tag. From there, just open your ringtone file in MP3tag and there is a tag section for "genre". Just name it "ringtone". Save the file, and transfer it to the ringtone folder of your phone.