My answer is based on my experience with Buddhist thought (about 13 years of study and 1-2 years of retreat)--when it comes down to it, it would not be problematic if there were to be a seventh sense door.
While Buddhism talks about the six sense doors (the regular five plus anything experienced purely in the mind [emotion, thought, etc]) which are said to encompass all that a human being could experience in this conditioned world, in theory you could throw in as many sense doors as you like and it doesn't make a difference--just be mindful, practice the eightfold path, and see dukkha as dukkha, and so on.
In another vein, having had some experience with what seems to be electroreception, it seems to fall under the realm of the physical or body sense door.
Regarding synesthesia, I have not heard about that in a Buddhist context, but I would extrapolate from what the Abhidharma says--each second contains millions and millions of mind moments, so no matter how fast or simultaneous anything seems to be, only one mind moment at a time ever happens. I haven't mastered meditation to the extent that I can verify that the Abidharma matches my experience, but from a intellectual standpoint, that would seem to be the answer, i.e. senses mixing is really just rapidly flowing sense perception.
Some interesting questions--all sense contact would fall on the wheel of dependent origination, so that would be a fruitful line of inquiry. And with regard to any sense contact, feeling, perception, etc., as Joseph Goldstein (American Buddhist teacher) once said, "Just notice what arises and don't cling." Can't go wrong with that.