TToday the way to do that isn’t point to point but rather join on a CMR. Spark user can dial the CMR sip uri and lync can do the same and Webex will handle the bridging between the two.
To be very clear spark meeting today which means hitting the call button inside a spark space or by dialing the uri assigned to a space cannot support Lync. Only standards based sip clients can dial that uri
In CMR however you can handle the transcoding
