Haven't tried to myself, but it's my understanding that you are unable to install FC3 into the Steamaps directory because of the registries, or something along those lines.
In my opinion, however, this isn't the biggest issue. If you could install it, how would you keep it up to date? the updates and modules in the Steam version of DCS:W are handled by steam itself, not a self-updater or Module Manager (Which might I add, is not included with Steam Version).
I believe the only way to keep it updated would be to uninstall and reinstall for every update, and given that there are normally 3-4 patches within a month of a major update, the StarForce is going to interfere fairly quickly.
My advice would be to just continue with the game and modules provided on the Eagle Dynamics website. As much as I want to use Steam, I'm happy with this arrangement:smilewink: