Adding a four way isn't hard. It's only the switch in the middle that's a four way, the wiring retains it's three (typical) conductors.

But yes, there are home-automation style switches/dimmers that will allow a switch to control other switches without them being directly connected.

Thanks ronnatalie,

I should have mentioned that I do not have good access to the existing 3-way switch wiring so that's why I was asking about wireless solutions. Hopefully I can find something that will just replace one of the 3-way switches with a receiver/switch, but I haven't found anything yet.

Leviton's Vizia line will do this.