Extending power adapter cable
5m of 20AWG is going to be on the order of 150mR. The potential being supplied will have to take into account the drop on both legs because the regulation point is in the head of the supply (no kelvin connection). So, at max current, drop is 3*2*.15=0.9V. Whether this works for your application is up to you.

I would splice it somewhere in the middle to make soldering easier. Use shrink wrap to protect the junctions and hold the discrete wires together. There is no benefit to replacing the entire 5m. It's still two splices.