Can an organization utilize this contract even though the majority of this task is no longer a bona-fide need? Also please confirm that is illegal to place a administrative support contract in place to have certain contractors by name in place.
As a general rule, contracts can be descoped by modification pursuant to mutual agreement or by partial termination for convenience. In either case care must be taken to assure compliance with the competition rules of the Competition in Contracting Act (CICA) as implemented by FAR Part 6. To determine the best course of action for your particular contract and to assure compliance with fiscal laws, you will need to consult your legal counsel.
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Regarding your second question about using an administrative support contract in order to have in place specifically named contactors, it appears that you want to have the contractor company assign specific people as identified by the government. This scenario sounds like it may stray into the category of personal services contracts which are carefully regulated under FAR Part 37.
Again, you should consult with your legal counsel on this issue.