Can a bayonet lug be added to a Bushmaster carbine?  From the pictures on the Bushmaster website it looks like the bayo lug is part of the front sling lug.  Is their bayo lug part of the front site base or the front sling lug?

Yes, the bayo lug is part of the front sight base.

You can swap front sight bases, but it is generally not recommended because there is risk of scarring the barrel when swapping these out, and they taper ping holes dont always line up correctly.  If you just *really, really* want a bayo lug, I would sell your barrel and purchase a new one.... especially, if you are considering having a gunsmith modify your barrel..... it will be cheaper in the long run.