I used calf heads for a couple of years (am back to plastics now). I would not recommend soaking the complete head, since that could make the head start slipping off the flesh-hoop, which would necessitate a retuck.
Also, I would not recommend adding anything to water that will be used for soaking a calf skin drum head.
Finally, if I wanted to reuse a vintage, second hand head and reseat it on a another drum than the original one, I would carefully, with a mildly wet sponge or something similar, moisten the head until soft to within say an inch of the flesh-hoop, taking care not to go too near the flesh-hoop and risking the head coming off it. Then, seat the head on the drum, put the hoop (rim) on and follow the general procedure for seating a freshly tucked head.
The other option would be a complete soak until the head comes away from the flesh-hoop, and then retuck it for the new drum.
/Magnus