Could I add butter to single cream to make whipped/double cream?
Yes, this is possible but you need a high-speed blender like a Thermomix or Vitamix to do so. There's actually a recipe on the Thermomix website:
http://www.ukthermomix.com/recshow.php?rec_id=29

Ingredients
250 gm unsalted butter
250 gm milk (full cream or semi-skimmed)
Method
Weigh butter in pieces and milk into the Thermomix bowl.  Cook 3 minutes/ 90C/ Speed 1.  Blend 30 seconds/ Speed 8.  This emulsifies the butterfat back into the warmed milk and makes a perfectly delicious cream.  Chill in the fridge 4 hours to use as pouring or spooning cream.  Chill in the fridge 8 hours or more to be able to whip the cream.

I presume you could tweak the ratio to account for the difference in fat between milk and cream to make your double cream.