To the OP ... yes it will work.
I often charge our RV's batteries faster when camping without hookups by using a generator to power both the RV's built-in converter and for powering a little inexpensive charger connected directly to the RV batteries' terminals. The currents from the converter and little extra charger do not directly "add up" however. Only part of the small charger's current capability gets added to the current that the converter is providing - not all the way up to what the small charger is capable of. So this technique does help some - but not dramatically.
