The thing is, it's sort of tricky this
Like I told a mate of mine:
Creating a backpack that can store stuff (like an ammocrate or a vehicle): Piss easy
Creating a backpack that you can carry with you on your back in the secondary weapons slot (like the BAS SATCOM): Piss easy
Combining the two: CHallenge!
I'm not sure OFP accepts you trying to give ammocrateness to a secondary weapon, but I don't have the time checking it out now (working on some other stuff + real life commitments) so you'll have to have a go at it first.