How can? Or can I at all use a 500K resistor (I've got some from Mouser,they exist) to make my single coils N and M read 250K from volume. And have the HB bridge read 500K. Would there be problems with the coil split on the HB bridge and M single in position 4? I know I can turn the volume down and do they same thing for the singles but I would rather not.
I'm a little unclear on what you're asking here. The link to your wiring shows a master volume and tone. With a SSH setup, pot values are always going to be a compromise. With a master V and T, a 500K pot may be a bit bright for the SCs; 250K may be too dark-sounding for the HB.
But adding a resistor is not likely to help matters much. The difference shouldn't be that big a deal that you can''t live with it. If it is, I'd say split the difference and use a 300K pot instead. These do exist although not as common as 250 or 500.