Yes it is! If you do a search on SK6 in this forum, you should find some instructions.
Basically all you do is put the 80mm fan on top of the SK6 turned 45-degrees so it sits on it like a diamond instead of square. The two clips on each side of the SK6 hold 2 of the diamond points onto the heatsink. (instead of the clips going down two of the sides of a 60mm fan)
Works great, doing it myself. There is a little air loss doing this so you want to go with a higher CFM fan. My 80mm Sunon 42CFM fan cools as well as my 60mm Delta 38CFM screamer. Noise drops significantly, of course.