That is just for ArmA 2, I knew you could do that. Equally, the expansions tab lets you add mods in the game itself with the Steam version of Arrowhead. However, Steam essentially uses a hack to graft the two together (they're installed to different places with Steam, but to the same place with retail or sprocket), which basically screws up mods. If you find a workaround for it, I would be very happy to know it
Sprocket is a nice way to go, and if you go retail there's good odds you'd want to use it to get the new DLC (which is really good by the way), though you can also get it through gamer's gate.
Main PC: Phenom II X6 1055T @2.8GHz, 8GB DDR3 Dual-Channel 1333MHz RAM, 1GB HD 5870, Win 7 64bit Home
Laptop: i5 540m @2.53GHz, 4GB DDR3 Dual-Channel 1066MHz RAM, 1GB HD 5650m, Win 7 64bit Home
In Arduis Fidelis