I just saw this post for the first time and probably the OP has done what ever he needs to do. But my thought was to remove what ever is on the feed thru lugs on the existing panel. Use the feed thru lugs to supply a 200 amp main breaker sub-panel. Install the items that were on the feed thru lugs to the new sub. Add the new circuits to the new sub. That way no new obsolete breakers are required and down the road the new 200 amp sub could be upgraded to as the new main panel.
The only real question does the existing service have enough capacity to add the new items he wants to add?
Coastal South Carolina