Retro fit heated seats?

Does anyone know if it's possible to retro-fit heated seats in a 2003-ish Passat ?

Yes it is, but it's a fair bit of work and pretty expensive unless you can find a second hand set. You'd need new covers with the heaters, and then you'd need to add the necessary wiring...
There were recently some new covers on eBay, so you could check there. But otherwise I think your best bet would be to look out for second hand seats complete - at least then it is just the wiring...