Creating a custom SQL Test Condition in Visual Studio 2013

I hope somebody can help me with this. I am trying to create a custom test condition in VS2013 using this guide.

I am able to create a custom condition but I am struggling with the property types.

I can create a string, int, or bool condition but I need to create a list of acceptable values, eg select a day of the week from the drop down list.

Does anybody know if this is possible and, if so, how to do it?

Many thanks.