To answer your question simply no a fork positioner cannot be used as a clamp. The main reason being there is no equalized clamping pressure on the fork tines to securely hold (clamp) the load. When you operate the fork positioner from the full in positioner towards the full out positionioner you will notice that one fork will move out faster than the other & the same when you move the forks inwards - reason unequal hydraulic pressure. A clamp will move the arms/forks in & out equally - there is a check valve system to help assure this.
A fork positioner is just that - it positions the forks (w/o load). Structurally the positioner is less substantial than the clamp - that's why they are more money.