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!
While helping out a cruiser climb her mast, I tied this knot between the two materials to allow her to hang safely from her wire halyard. The knot is a combination of a sheet bend and a figure eight in a bight. An important thing to do with this knot is to certainly secure the bitter ends. I attached the shackle of the wire onto itself and the bight of the rope had the tail passing through it. These added features are merely present so that if the knot were to slip, it would slid onto the attached bitter end and stop moving from there.
Naturally, since this knot is new to me and created on the fly out of necessity, our cruiser friend who was in the harness and at deck level was encourage to drop from her feet to a seated position to test the knot and verify that there is no slippage or other concerns that might occur while she was aloft.
While creating a new knot, be sure to test it at its desired function, and incorporate some method that will facilitate ease of untying after the knots intended purpose has been completed.