nope when u cut one of the wire of psu and extension that pretend warranty will be void
OP had a prebuilt Dell PC in which he is swapping out the motherboard. That in itself very much voids whatever warranty (if any) might be left on the Dell system. Splicing wires to extend them is not going to make much of a difference.
As someone mentioned, since the PC originally had an LGA775 board and Dell has not sold LGA775 XPS-series PCs in years, the warranty on the PC and most of its components likely expired a long time ago.