Sure it can be done, but it is very expensive to go OE. BMW sell replacement seat covers with the heating elements sewn in. $$$$. Labour involves recovering the seats, adding the wiring harness, etc.

Aftermarket solutions are available.

Lots of threads on this, including experiences of others who have do it. Do a search on heated seat retrofit. Here is one post on the subject, not specific to the E70 (but the concepts are similar)

http://www.xoutpost.com/780236-post7.html

And here is one specific to the E70:

http://www.xoutpost.com/bmw-sav-foru...ted-seats.html

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue