If the cells did not get wet, then they should be fine for use. If they did get wet, then they may have issues later from corrosion, unless they were quickly dried out thoroughly. (water gets under the caps on the + end, for instance) You may not see a voltage problem from this initially...but later on problems can occur.
That said, if the cells really are ok, then you may be able to use them "paralleled" with the cells in the new pack, to double the capacity of that pack, and use that pack's electronics to monitor old and new. But if the cells aren't ok, then they could cause problems with the cells paralleled with them.
There is a possible problem with usign these cells "naked" as it were, or with an alternate BMS: Most of these OEM ebikes will only work with their OEM batteries, as the controller/etc talks ot the battery to get status, and if it cant' get it it won't operate at all (or it only works in some very basic mode, maybe not ridable, for troulbehsooting or something).
Only way to find out if that's true is to try the old pack on the bike but with the + and - of the actual battery inside connected directly to the output + and - pins on the case, completely bypassing the electronics board.
BTW: I think that OEMs that don't completely waterproof critical expensive systems like batteries are making bikes that are really useless for real-world applications. Even here in Phoenix, AZ, middle of the desert, when it does have severe rains, I have to ride in them anyway, and if the bike couldn't take that, I'd be screwed. I've ridden thru one flooded area that submerged a significant amount of the trike, past the electronics and battery, and while I didn't have it waterproofed, I was lucky nothing failed from it. It could have stopped me cold, and destroyed stuff I couldn't afford to replace....
I expect a lot better from OEMs.
