Domains transfer can be allowed or disallowed as a system-wide option, in store settings.
However, if you allow domains transfer in general, you still can narrow the set of domains allowed for transfer by selecting particular TLDs which transfer is allowed for. You can select TLDs allowed for transfer for each domain registration plug-in.
To select TLDs allowed for transfer, go to the Service Director - Domain Manager - Plug-Ins. Select a plug-in and then click the Prices tab. Select a TLD from the list that appears and check the Allow transfers box, set transfer fee.