No, more player slots can't be done on that server - most maps (apart from cbctf4) just don't survive more than 16 players.
What I am thinking of, however, is making a system to dynamically start a second server when the first one is full, and "phasing out" the second server once it gets empty enough... like, once 14 players are on the server, a second one will be started... and once the total number of players goes below 10, the second server gets instructed to redirect players to the first server and shut down at the end of the match.