.rar is a packed file, something similar to .zip
So you don't have to convert .rar to .pbo, just unpack the .rar file and you'll get .pbo (and usually some readme.txt and maybe other useful/interesting stuff).
Use google to find a unpacker that can handle .rar files
personally i suggest 7zip which can handle .zip .rar and a few other packing formats.
You can get it here