I assume this is in your own home and is not mission critical stuff and that pulling a new wire opportunities are past. If that is the case, I would splice the wires and solder them keeping it nice and neat. Make sure to keep the twisted pairs twisted. I like the heat shrink insulation over the top of electric tape on each wire. Keep it neat with minimal tape that is cut not torn and don't strip back more insulation than necessary (less than the width of a piece of electric tape).
If it doesn't function properly, you can always redo it since it sounds like this splice will not be buried in a wall. The nice thing about a splice is it won't require any bulky stuff to mess with your wire routing. I did this once before and it worked perfectly though it was only a temporary deal for something I wasn't leaving in permanently and wanted to just use the wire I had at hand.
Other options include crimping on connectors and using a female/female junction. That is a little bulky but not too bad. The splice junction box listed above is the most professional, but more bulky and might cause wire routing issues. Of course the truly most professional is too always leave plenty of wire!! <_<