There is probably a proper solution. I have seen speaker wire extension cables but the ones I have seen are all a very small gauge. Smaller than I would want to use for my distances.
However, if I were to have to do this today with my spare parts pile... Most banana plug receptacles are removable so they can be added to differnt types of wall plates. In your case, you just don't need the wall plate at all and any plastic clips could be removed if they are there.
Put the recepticles on the speaker's wire and the banana plugs on the other wire. Connect as needed. Ensure you get a banana plug that can be tightened for a secure fit.
Add a little colored shrink wrap to where the wires enter the plugs & receptacles to make it look a bit less hackish.