Yes, those have always been possible, but as I mentioned, there needed to be a Mint fare filed for the city pairs.

For example, when Mint first rolled out, Mint was only offered out of JFK (to LAX and SFO). However, I could book a Mint + Core connection on the same ticket, online, out of Boston at that time, because there were Mint through-fares filed between Boston and SFO/LAX. So, what I mean is, if there's a through-fare filed for a common logical connection, you can book it all on one ticket online. If you're trying to book between a city pair where they didn't "think of" filing a Mint fare for, then not even an agent can do it (piecing it together does not work).