Well, unless you plan on using this as a stepping off point on upgrading the rest of your system, I wouldn't bother. As yoyo said, the crossover on the Z-5500 system is set really high, meaning only frequencies above 150hz (or somewhere around there) are sent to the satellite speakers, so you wouldn't get a benefit of having the fuller range that the R50 can provide because they just aren't getting the full range of frequencies sent to them.
If you're planning at some point though to replace the Z-5500 control pod and sub and whatnot with a receiver and dedicated HT sub, then yes, it would work in the meantime.
Edit: Bah. Yoyo beat me to it again