I don't know anything about extension caps for the fork tubes - but I supposed it is possible.  On a Cafe' conversion you drop the front forks about 2 inches - so if you have the top clamp that RYCA now offers you could attach the clip ons to the fork tubes that are extending above the top yoke.
http://shop.rycamotors.com/aluminum_triple_tree_clamp_suzuki_s40_savage_ls650...

Or.....you could eliminate the risers and put the clamps down on the stock yoke, and then install a handlebar that is comfortable for you.
http://suzukisavage.com/cgi-bin/YaBB.pl?num=1399388372

I have my bike modified and I have dropped the fork tubes, and currently my clip-ons are under the yoke.  When I get older and tired of bending over so much....I can relocate the clip-ons above the yoke.

I found this website that I cannot find anymore (which sucks). They have this caps that replace the stock caps on your forks to lift up the front end and make it look more like a chopper. They go from 2 inches up to 4 inches.