[Mobhack]

In Mobhack, open the OOB with the weapon you want to copy from. Navigate to it on the weapons tab, and then use the copy function. That weapon is now in the paste buffer.

Now open up your target OOB, go to the weapons tab and find a free weapon slot (if there is one - if not, you are stuffed). Paste the weapon you just copied into that slot. Save weapon data before moving off. Now open your modified APC in the units tab, find the cannon (probably slot 1) and change its ID to the slot number you used to paste the copied data into. Change ammo to suit. Save OOB and exit Mobhack. Job done (though you should run the points calculator on the resulting OOB).