Any way to create a multisig address from Bitcoin addresses (instead of public keys)? Any way to create a multisig address from Bitcoin addresses (instead of public keys)?

If not, why not? What would the problem be?

EDITED - Thanks for the answers. This is for an application that generates multisig transactions with escrow for external users. Of course anyone who controls an address can find the public key, but that's probably beyond many casual users (or too much of an hassle).
While you can't generate a multisig address directly from a number of non-multisig addresses, you can easily get their public key on a single call. In bitcoin-core you have to call validateaddress and then retrieve the pubkey field from the response, which is the hex value of the raw public key that you can then use to generate your multisig.