Hi,
Nameservers wouldn't normally be changed when transferring a domain, so there should be no downtime anyway.

However this is already possible:
Navigate to the order details page (Orders > List Orders > Click Order ID) and then UNtick the "Send to Registrar" option before clicking the "Accept Order" button.

Then when you're ready to initiate the transfer process later, use the Module Command buttons under the client's Domains tab