It is possible, I've done it. You will need to purchase a Quintim FXS gateway AFG-200 (2 port FXS, they have up to 8 if you need). They have firmware that can upgrade the unit so it will show up like adding a phone. Works great, have not had any issues with it yet.
DLink moderator, I apologize for listing another vendors product, but the fact is they are the only vendor offering an FXS gateway for RP at this time. If anything, this greatly expands the functionality of any RP system and you never have to worry about Quintum selling RP systems (gateway only vendor).