Alfa,
When I joined the RNZAF I had to be free of my UK Reserve commitment. I wrote to PMA, enclosing the offer of service from the RNZAF and they promptly wrote back freeing me of a Reserve commitment whilst employed by the RNZAF. Should I leave the RNZAF then my Reserve comitment will be re-implemented.
If its a civvy company you are going to try the same gambit. If you are going to another military, you now know the answer.
Cheers