Can voluma actually be injected into the buccal fat pad itself?

Yes Voluma can be injected here, but it is highly unlikely that you would want this area treated in isolation - it can take a lot of product to see a meaningful result in this area, and it is very likely that by using Voluma in other areas as well you will see a much better overall result. 
As far as technique is concerned this is an area most suited to treating with a cannula because of the risk of damaging surrounding structures with a needle, but this is down to preference of the physician.
You should be able to get a good result without having prominent cheekbones - this is something you would need to discuss in detail with your physician before committing to treatment.

The loss of buccal fat is best treated with a hyaluronic acid filler like Juvederm, Vollure, Refyne or Defyne in the subdermal plane for volume replacement.  Hyaluronic acids used in this plane may stimulate collagen growth and remodeling.  You would not chose a product like Voluma because it is not made for this mobile area.  It would not be wise to place any filler in the buccal fat pat itself.  Be sure to chose an experienced aesthetic physician for your treatment.  All the best, Alexis Parker MD