Well yes - and no!

If you take it to a stealer, they want daft money to do retrofits. But as quite a few people have found, actually doing the job yourself isn't always that difficult or costly - take doing Nav retrofits in E46's etc...

If as the heated seats retrofit notes appear to show, the heating pads aren't already installed, then the cost and difficulty would make it unviable. If as it appears by looking at the seat cover parts in the ETK (which doesn't actually list seat heating covers as seperate parts) the seats are ALL fitted with the same covers which actually have the heating elements in, then possibly all that is required is a new switch unit and maybe some wiring (or perhaps not).

There was a comment a while ago on the bmwcarmag forum about seat heating in E46's where someone had done the retrofit as the heating pads were already in the seats - that just needed a replacement switch module, fuse and some link wiring. I am hoping the same is true for the X3