A patch doesn't necessarily apply to a file type. A patch simply contains all the changes and it is made by comparing the file no1 (ie the original game) with the file no2 (ie hack).
So why would you want to do this with the rar file. Can be done obviously if you want to change the rar file-1 into rar file-2. But this is unusual, since it has nothing to do with playing or patching roms. It will simpy change one rar file into another rar file.
I think you want to unzip the rar first?
This patcher can create patches by comparing Any 2 fileshttp://www.romhacking.net/utilities/917/
It can also patch anything to anything, but this doesn't mean that the game will work. For instance if you patch one of the IPS patches on this site to any RAR file, the game will not work (obviously).