Yes, it's quite possible, You should just hook the initialisation function of the engine and replace the real number with desired one. But you will also have to hide these extra players and extra slots and change some cvars and values, or you will get banned by the hoster. You should also take into account that the server may not have enough resources for more players than it can hold now. Some hosters, as the one I use, have got protection systems for preventing such fraud things. Even unsuccessful attempt can lead to ban and you will have to look for another hoster. The game isn't worth the candle