Make function payable for a specific ERC20 token?

An ERC20 token is just another contract. The ERC20 standard gives you two functions that work together to help pay a contract: approve() and transferFrom().

If the token contract is called "token" and the contract you're trying to pay is called "store", the process looks like this:

1. The user calls token.approve(store, amount); This gives store permission to transfer amount of the user's tokens.
2. The user calls store.buy();, which calls token.transferFrom() to perform the actual transfer.

The buy() function might look like this:

function buy() external { require(token.transferFrom(msg.sender, this, amount)); // having now received <amount> tokens from the sender, deliver whatever was // purchased, etc. }