I am attempting to convert my above ground Thermospa into an in-deck spa and need to extend the wiring for the spa side control and temp/hi limti sensors etc about another 5-8ft.

I also plan on relocating the pumps and blower in an attched pump room.

Is extending the wiring harnass a problem?

nope, no problem

Ok then I have been searching for a 14 pin connection extender for a Balboa control and do not see it listed in the Spa parts,

Or should the wire be spliced?

I am not sure where they can be had. I have one, but it came with the spa. I would suggest contacting either balboa or the spa manufacturer. Perhaps even some spa dealers. I believe it's just a ribbon cable extension that you are looking for.