Yes, there are a few ways to leverage your existing SIP phones (Cisco, Avaya, SIP softphones):
SmartSIP from NET – allows you to connect your existing SIP phones such as Aastra, Cisco, and SIP softphone (X-lite, etc) on a Mac to OCS
SmartSIP Mac voice to OCS demo (Evangelyze sold SmartSIP to NET) here.
AudioCodes new SPS (SIP Phone Support) for Microsoft is an add-on for their gateways which allow for ANY SIP endpoint/phone to connect to OCS – Audiocodes 300HD SIP phone, – this also includes Mac SIP softphones to OCS
Just so it is clear, these solutions are not going to provide a full OCS voice/presence experience that you would receive with an OCS optimized endpoint so this is not a recommended long term solution for connecting to OCS. These solutions will however provide a temporary bridge when transitioning from your legacy PBX and legacy SIP phones to OCS and OCS optimized endpoints.