You certainly can upgrade to a long-arm kit from a different manufacturer.
I have heard many many positive comments and reviews of the Iron Rock Offroad "Critical Path" long-arm kits, and they also have long arm 'upgrade' kits that can be used with existing lifts from 4"-8".
Be advised that the kits to look for from IRO are the ones labelled as 'critical path'. These use a special Y-shaped control arm, which are the ones garnering favourable reviews. In a nutshell, they provide incredibly well-mannered performance on-road, and even better flex performance off-road
Personally I've been looking into their XJ kits, and I know they make them for the Grand Cherokee ZJ and WJ also, although I am not sure what other jeep models they have their critical path kits available for.
You can check out their website here:
http://www.ironrockoffroad.com/