If your existing seat clamp (seat collar) has flat, parallel clamping surfaces on both sides, and the hole is at least 5mm in diameter, then yes – your existing seat clamp is compatible.
However, if your clamp does not have the two flush surfaces that Pitlock skewers require, the integrity of your locking seat skewer may be compromised without a Pitlock seat collar. This is commonly the case if your existing clamp/collar has an integrated quick-release skewer, uses a proprietary bolt, or has a hole that is less than 5mm in diameter.
To purchase a new seat clamp, measure the diameter of the seat tube itself. The sizes of Pitlock seatclamps (for exampple, 31.8mm) will refer to this measurement: