It looks like the only way to add a domain list to a property of a registered spatial feature is to define a foreign key in the database before registering the spatial table.
It would be good if you could add a domain to a property that is not defined using a foreign key and lookup table in the database. I.e. adding a Domain list option under the Add menu item of the pop up menu for feature properties.
Currently the only way to do this is to Hack the XML in the Project/feature xml files.