Most likely you cannot do this. You can only bill a facility fee if you are licensed, credentialed and enrolled with the payer as a facility provider.
But I'm not sure what exactly you mean in saying the practice is doing this 'at their own expense' - what expenses are being incurred by the practice? Is the practice paying the facility for the use of the space and the facility staff's time? This would be a very unusual, and possibly non-compliant, arrangement with a facility. The facility should be the one providing all technical costs involved in a procedure being performed on their site. The only expense the provider should be incurring is the professional service, unless the provider has a designated suite or office inside the facility for which they are responsible for all operational costs. If that is not the case but your provider is paying some of those expenses that the facility should be covering (i.e. use of equipment, drugs, supplies) then the provider should be seeking reimbursement directly from the facility itself.