As a former LM, i'd like to point out adding a boom to the back end of a cargo aircraft isn't trivial. Assuming it's a two-piece door like the C-17 and C-130 (ramp and cargo door), yeah theoretically you could add it to the cargo door, which opens up and into the upper fuselage; but obviously you'd have to have the fuel lines, necessary hydraulics/control wires etc. attached via flexible lines to the door, and still have enough clearance for the door to retract up against roof of the aft fuselage. I can't speak to the -130, but on the -17 that's not a whole lot of clearance up there, and the door is already used for ramp toe, cargo gate bar, roller tray and centerline seat stowages, along with other small items like the engine core hydraulic hand pump (if Mx actually put it away where it goes, and hasn't been lazy and left it strapped down on the cargo floor somewhere), engine covers, etc. And once the door is open, how far does the boom attachment protrusion stick down? You've now lost a bunch of vertical clearance for pallets, or vehicles driving up and over the ramp crest (which is why mil cargo aircraft have upward-bulged rear fuselages, so tall items like an 18-wheeler trailer or a Chinook helicopter or whatever have upwards room to clear when going across the ramp crest).
The ramp, well on the -17 it's used as a loading and lifting surface (up to 40,000 lbs, and 4 pallet spots in Logistics bias, two in ADS), and there isn't much room to work with. The ramp is obviously intended to go down to the ground so if you somehow had a boom hanging off the bottom of it, it wouldn't be able to do that anymore, and even if it strictly went to horizontal for pallets only there still isn't a lot of clearance underneath, like 3 feet, plus you have the boom sticking out however far behind meaning a k-loader likely can't drive up to the back of the ramp for cargo transfer. Also you'd lose a point of ground egress as the ramp blowdown is somewhat negated now.
Underneath the belly, between the mains yeah I suppose that's a possibility, it's just a low ground clearance (2-3 feet) as cargo planes typically sit low so they can be unloaded at truck bed height, unless you want to do a bunch of hydraulic jackscrew variable-height bullshit like a C-5 and then wonder why the fucker breaks all the time.
The C-97 had a bomb-bay style cargo loading door at the aft fuselage, which looks like steep sketchy shit for loading vehicles, and for the KC-97s the boom pod and boom mount was a plug that went in, in lieu of those doors.
Point of all this is I'd imagine the -390 is really similar to the -17 and -130, and I think the A400M is as well. Mil airlift moved away from the 4-5 door system of petal and bulkhead doors like in the -141, FRED, An-124 and Il-76 for a reason (and it's not like those would help in this situation), and other than manufacturer's concept art I don't think anyone has ever really put a boom on the back of a mil airlifter. It's not a casual undertaking, and I think once you've done that it's kind of a one-way street, hard to imagine it still being an effective airlifter anymore even if it's a secondary ability.
Now that's all for the traditional boom grafted onto the aircraft approach. If you could have the whole tanker setup be a substantial palletized system, with the boom extending out through the cargo door/ramp opening and then down, and able to be entirely sucked back in to close those doors, that could work. But now you're depressurizing every time you give someone gas.