Thanks OldMech.
Yes I had the experience observing a prospective customer that was unloading a container of marble slabs & a lift with a fork positioner & was using the forks to stabilize the load from side to side. The slab shifted to the right side, the right fork moved & in a blink of an eye later the slab hit the ground - no one hurt - change of undies was necessary for the operator. It is just amazing how many little pieces of granite are in one slab.
But I (a sales type) being in the right place at the right time seized the opportunity to "solve " the issue for the customer with a lift of greater capacity fitted with a s/s clamp & a granite/slab handling attachment (fork mounted) - commission check was a bit larger that month.

johnr is correct that a fork positioner cannot be used as a fork clamp.

However, there are attachments that ARE fork clamps that look like fork positioners.

Get the attachment model off the data plate and check with the manufacturer to determine exactly what the attachment is capable of.

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.