Once you have built a selection list on a Smart Entity you are unable to edit or add to this list like you would with a standard entity


In Administration -> Customisation -> Translations enter one of the values that already exist in your list into the UK translation box

Select the green caption code hyper link

This will show you some of the data needed to add a new selection

If you are adding a new entry  then  on the previous screen select New - > Translation

The Caption Family and Caption Family Type will be exactly the same as your exisiting entries

You will need to add your new caption code and translation 

More information:

The caption code must be unique to that look up family, adding the caption code as the translation makes it easier to find later ( Avoid using special characters in the caption code)