[url= http://www.merlincycles.co.uk/marzocchi-and-rockshox-suspension-forks/rockshox-suspension-forks/rockshox-reba-s-l-2010.html ]Reba SL 100mm[/url]
Adjustable to 120mm or not?
I would say yes, they have a plastic spacer inside which needs to be removed to extend the travel to 120mm.
Yes just remove that plastic spacer
You need to check inside the left (disc mount) leg to see if you have a black 'all travel' bobbin spacer in there. Some OEM Rockshox SL's aren't adjustable, some are!!! Aftermarket should be. You need to have a peek. Loads of little videos on SRAM site or try YouTube.
Well worth doing-makes a big difference. Mine were 2009 Merlin forks-OEM.
Correction-some OEM aren't adjustable to 120mm (100mm is a max)
My 2008 Reba SL went up to 115mm no probs - just removed the spacer, was very easy actually (although to be fair I only watched while my friend did it!).
(They're still up at 115mm and for sale if interested - sorry to be cheeky!)