Its unlikely this is a good idea. The salt cell is sized on amps. Logic suggests, and I am not familiar with this specific equipment, the RJ30 is a smaller control box with less amp output than the RJ 45 cell. I think the cell will run, but it wont be outputting the RJ 45 amount it will be outputting the RJ 30 amount. However it is an equal chance it wont run at all as the manufacturers may include a circuit to prevent this.
HTH