I doubt if adding one more sprinkler to a zone would overload the capacity. I assume you have PVC pipes and you are going to add the new sprinkler somewhere in the middle of the zone. There are a couple of ways to do this. If you can dig up enough of the line to lift it up a few inches, you can just cut the line and glue in a tee. To do this you would want to cut between 1/8" and 1/4" from the pipe to allow the tee room to slip onto the pipe. If you can't get enough play in the pipe to do this, you can use a repair coupler to reconnect the line. These couplers work great, just cost a bit more than a plain old tee. The advantages with this method are less digging and being easier to make the connections.