NetJets have a contract with Signature whereby they'll use Signature at just about every airport that has a Signature on the field. Prominent exception is TEB, but that has to do with Santulli being loyal to the FBO who took a chance on him and his company before NetJets became what they are today.
The other exception will be if the customer has a specific FBO preference, then NetJets will abide by the wishes of the customer.
At HPN, NetJets will be opening their own new hangar, probably at the end of June. It's Seagram's old home (Hangar 6), and has been used by NetJets for maintenance since the 3rd quarter of last year. Reconstruction and upgrading will continue through June, at which point the hangar will also begin to serve as NetJets' arrival and departure point at HPN.