Hi,
For your case, it is not supported. However, JG client can join a conference if (a) dial to a conference bridge (b) or, UCM endpoints create an ad-hoc conference with other SIP endpoints.
If you are using another pair of VCS-E/C and create a SIP trunk with UCM, you should be able to create a conference between 3 parties : JG endpoint, UCM endpoint and VCS-registered endpoint. See case (b) above. The network will looks like JG_client -> VCS_pair_1 -> JG_server -> UCM - VCS_pair_2 -> VCS_endpoint
Btw, JG will not behavior like a rich-SIP endpoint, SIP REFER is not supported.