Just take some bioballs from the 55 and move them to the new sump. You don't need a lot. Take a handful and lay them on top of or in front of the new filter's biomedia. Take two handfuls if you can, just be careful not to disrupt the existing one. 3 months isn't really that long for a new setup. I'd be a bit conservative. Shouldn't take much media. I've cycled many times in 7-10 days using this method.