Not sure if I'm understanding you correctly here, but with the current gear system, if you only see a weapon, you can take that and equip it. If you want attachments for it, you can take them from anywhere you can find them and put it on the weapon.
And if you're looking say, at a crate which contains the weapon you want, with attachments on it, the tooltip and icon of the weapon shows them attached, and you can also take this.
It seems a very logical system. And to simulate realism, since that is what the game does, it would break this element if you could just pick up a weapon without any attachments and just magically select what attachments you want and they'd pop out of blue air.
I believe there's a loadout mod for the Alpha, and though I haven't tested it yet, I believe the one for Arma 2 allowed you to atleast save a loadout, and then select it even if you were nowhere near the items the loadout consisted of. That might be something you could look into as well