You might obtain a clearer view of the available vacancies, including the 'Sponsored Reservist' requirements, at
Jobs AirTanker .
Clearly it makes sense to adertise for some existing A330 aircrew to join the company and pass on their experience, although from what I've heard the vast majority of pilots will be drawn from within the RAF's current AAR fleets. Assuming, that is, they don't all leg it to the airlines.....
When the VC10K fleet first started, all the captains were either ex-Victor/Vulcan AAR or ex-VC10. Some were both. The first co-pilots came from a variety of backgrounds, although it was a couple of years before the first ab-initios arrived. The synergy of this mix led to a sound operational method being evolved, without a legacy of "We always did it this way on the...." dominating. The A330MRTT fleet will probably adopt a similar strategy.