You should probably have a separate client to use to have the other keys. The server generates a key and imports it immediately. Users are asked to supply the required amount of keys, send money, paste the txid, and enter an address to send the money back to. Then they need to sign the transaction - the software will automatically sign the transaction it generates, but you could leave this out if people wanted to actually do escrow with it.
It only works with bitcoind/qt. If you get multisig transactions to sign in any other client that only has 1 of n keys please let me know!
Problem. Your server could be hacked and used to sign transactions that are sent to wherever an attacker wants. Most users will give you keys from their wallet.. Imagine taking the private key from a users android app, and you get hacked. All their phones coins will get spent, just because you wanted to sign the transaction for them. You should not have that ability if you're trying to implement multisig - the user needs to sign for themselves.