I'm not sure if you've since sorted this out, however I've just been through the same situation.
I bought my HP4 2nd hand from Perth, Australia to Canberra.. It was only once it arrived that I noticed the lack of heated grips (I thought they were standard but apparently a A$200 option). I can't say I was surprised though as it's twice as hot over in WA than it is on the eastern side.
ANYWAY - My dealer advised the same thing in that they couldn't be retrofitted. A little more investigating and It turns out they can - and I have! A good sign is that you have the heat indicator on the dash. A sure way to tell though is as like someone above mentioned, pull the left grip off (easier) and you should see a two prong connector inside the LHS switchgear.
What you will need: Full left heated grip, Full right heated grip, RHS switch gear with one that has the heated button (top half only needed as the bottom casing is the same).
It's as simple as doing a 1-for-1 replacement, with the only difference being you'll be connecting the two prong plug on each side. The right hand side one was a bit tricky to get the cover back on as it's a tight squeeze with the new plug / cables in there.
THEN - They won't activate (nor appear on the display) until you get it coded by a dealer. My dealer said their system didn't recognise the heated grips on the first attempt and then the update took around 45 minutes to upload.. Either way - I now have toasty warm grips.
Mine is a late 2013 HP4 and I got the grips / switch off the other s1000rr forum... Grips / switch were around $300 AU total (900 new) and coding was $70 AU.
Good luck!