You could of course drag the beginning and end of your rex file, down to one or more tambourine hits, then copy and paste the file wherever you need, to get the desired pattern.
Maybe someone knows better, but doesn't Sonar include a vst instrument called "RXP", which is a rex file player. You could load the file into RXP, and play the new pattern on your keyboard. CHeckout this thread:
http://forum.cakewalk.com/How-to-use-RXP-Rex-Player-m667368.aspx Another option would be to throw the rex file into a sampler which support rex files, like Shortcircuit, which is now free, and can be found here:
http://www.vemberaudio.se/shortcircuit.php You could then program a different tambourine pattern.