can't you parametrize the hole diameter and the fastener diameter independently or concurrently? that's one way. I don't know how it works in Mechanica, but some FE fastener models automatically specify fastener diameter when you create it--whatever hole the fastener is assigned to, the FE software automatically assumes the fastener and hole are
'neat fit', that is, the same diameter. To specify a fastener hole clearance or interference, you might specify that someplace else like the Load window/interface.