First off, I want to emphasize that you should never climb a mast with a wire halyard, or a wire-rope spliced halyard.
Now, if you have a wire halyard and you need to go up, then you need to make due with what you have!
If your halyard isn't long enough to reach your harness, now you need to attach a length of line between the end of the halyard and your harness, that way when you are finished, you will end up on the deck and not suspended a few feet above the deck when you are tired and want to get out of the harness!
Wire is dangerous to tie knots in because it is both slippery and brittle. If you bend it too tightly, it will crimp and kink, damaging and weakening the wire strands and leading to its eventual breakage. Keeping bends open will also make it easier for the knot to slip and come untied. Thus you encounter the dilemma of tying a knot in wire and rope!