Ok, couple of things. First of all in the picture you've provided the FA-MAS has had an M16 magazine conversion (note the slightly bent magazine shape) so unless you have it upgraded to take an M16 magazine, it will have a magazine that looks like this:
Second, the FA-MAS can be quite heavy, I myself have a FA-MAS SV and with a scope, mount and extra mag its even heavier so a sling can be a life saver especially if you play longer games. If you were to purchase a FA-MAS SV you may be able to buy seperateley the pistolgrip, hand grip and bipod off someone (perhaps with a broken FA-MAS F1?)
The parts could easily be attached as the FAMAS-SV and F1 bodies are identical, only the pistolgrip, handgrip and bipod are different (as well as a few other minor cosmetic differences like the barrel design on the F1 is slightly different - it has the screw notches on it)
You may be able to retrofit something onto the bipod to hold a sling, but it could be hard - it MUST be able to support the weight of the rifle or your screwed. Now the other thing you could do is buy an F1 and just tie the sling around loop hole on the butt and around the carryhandle?? - it could be done. Its a bit of a bodged solution but it would work.
If your a sniper though and you want a FA-MAS as backup i wouldnt recommend it. As mentioned before its fairly heavy and somewhat unwieldy. Its a great assualt rifle, probably the best for price, but in buildings and CQB as a backup its to large to use effectivley. If you are a sniper and want backup have you considered an MP5K? The smallest fully automatic AEG available - its very good indoors and in CQB, the other thing would be a shotgun - in my team our sniper has a shotgun for backup to, works a treat in CQB.