Setting aside the music part of this problem... VRC6 doesn't play nice with PAL.
VRC6 uses a CPU cycle IRQ counter that simulates scanlines by counting 113.6667 cycles and treating that as a single scanline. The problem is, on PAL, scanlines are ~106.5 cycles long. So EVERYWHERE the game uses IRQs (which, in some places, is 3-4 times a frame), the IRQ counter would need to be readjusted, and it's somewhat possible you might not be able to get the timing exactly right, leaving you with some screen jitter.
So... yes. It is possible, but it'd be difficult.
Though part of me thinks it might even be easier to retrofit the Japanese version and convert that to PAL rather than trying to convert the PAL version to be VRC6. But that might be more difficult than I'm imagining, depending on how much the PAL version changes logic-wise. This is probably where I'd start. Maybe compare the US version and the PAL version to see what the changes are, then try to migrate those changes over to the VRC6 version. Then after THAT, go back and try to clean up the IRQ stuff.
But yeah this is a big job. Probably not one I'd recommend for a newbie, but I've been surprised by newbies before.