Working with inbound and outbound interfaces in studio is same as for MPT and SPT. You can create new interfaces in EPT and override the existing interfaces from MPT and SPT.
One thing to note however is that in case of server side message processing, message processing is handled by default using a "Value Chain Admin" context. Because this user is not Enterprise-specific, it will not be aware of any interfaces you have added to the EPT.
To upload data through EPT interfaces, you will need to load MsgProcessingContext.