There is no official support for other handsets specifically. However all the KIRK controllers are GAP compatible so you should be able to subscribe the handset. However trying to support handsets other than Polycom KIRK handsets is virtually impossible due to the sheer quantity and variation in software releases on these handsets.
Today the range of handsets from Polycom KIRK is exceptional and for all environments and budgets. I can't really imagine a reason why I would use an alternative handset other than with a large deployment that has a large investment in another manufacturers range of handsets.
Your Polycom reseller should be able to help source the best KIRK handset for you.
ICON - UK SpectraLink Wireless Distributor icon-plc.co.uk