The hoverboard Packs will come with their own BMS. You can parallel them to get power out of them, but it might be better to charge them individually.
Alternatively, you could just use 1 bms (from anyone of the packs) and run hard wires to tie them all together as 1 "battery."
Most hoverboard batteries are going to be 10s2P, or about 4400mah at 36V nominal, 42V fully charged.
If you were to cut the BMS off of the other backs, and tie the wires altogether to a single BMS, as well as tie the power and grounds together, you would essentially have a 36V, 10S2
P battery n(4400mah) battery, where n is the number of packs you tie together.
Assuming you can find a 10S charger that will charge at 3-4 amps, it would take about an hour per pack to charge.
So if you string 5 together in parallel, and rely on a single BMS, you would have a 22 amp hour battery, and it would take somewhere around 5-6 hours to charge at 3-4 amps.