If you want to create a second property (instead of replacing the property itself), you'll want to create a new property in the main code file so that it does not get overwritten when the partial class is re-created again.  The easiest way to do this is to copy one of the fields from the designer file, paste it into the main code file (attributes and all) and modify it the way you want it.  Then, just override the GetCustomBindablePropertyDescriptors() method and return the property descriptor for that property.  (You should be able to search either the help or the forum for "GetCustomBindablePropertyDescriptors" because it might show you some more information and show you what happens when you don't provide a descriptor ).