Miscellaneous Charge encumbrance for Third Party Vendor

We want to encumber the cost we are paying as miscellaneous charge for a third party vendor. For Merch Vendor, PeopleSoft encumbers the cost but for Third party it doesn't.

Problem is that we are incurring huge cost as misc charges for such Third party Vendor and if we don't encumber such costs, we may end up spending a lot and not realizing it until we voucher the invoice of third party. Please let me know if we have any workaround for this scenario.