This is just corporate speak or legalese to say that first they will cover as many positions as they can with volunteers and the rest will be filled by designated staff.

So I would forget about this particular section of the sentence and translate the whole sentence something like this:

Volunteers will be used to cover as many positions as possible in the work teams needed to carry out said task/job/work, with the remaining positions being covered with other members of staff, appointed by the company on the basis of the company's organizational needs.