Yes you can fit a clasp to any strap with a buckle. A clasp only strap will need a buckle prong slot cut if you intend to fit a buckle.
Look in ‘Description’ section under images at listing page to find size of buckle /clasp end of strap before purchasing a clasp (or buckle)
Add strap to cart > navigate to accessories > add correct size clasp to suit your strap to cart. A check box will appear asking if you would like us to fit clasp to purchased strap or not. Proceed to checkout.