The client is centrally controlled. They could extend it with the next release if they wanted. Everyone would have to use the new client though.
No, they could not, the total number of generated coins is based on the formula that governs block generation, as such any change to this formula would require the creation of a new genesis block and it would invalidate all old blocks. Essentially starting the Bitcoin network again from scratch, you could not transfer Bitcoins between the network with the old formula and the network with the new one. This is a core function of Bitcoins, no matter what you can not make more than 21 million without starting a new network.