Probably depends on the kick unit.
Both my vamo and sig zmax are supposed to work with 2 x 18650 stacked batteries therefore they can accept 6.6 to 8.4 volt input.
So maybe you can make use of the vamo v5 module which should therefore accept it?
Otherwise there is the Sigelei drop in module, which, if it accepts the same inputs as the sigelei zmax, should be able to do it too:
Oh, but wait... the question was whether it would be safe! No; the kick won't make stacking batteries any less hazzardous.