How add an option to a dropdown menu bound to a secondary data source?

[pmbarnes]

I'm trying to add an option to a dropdown menu that is bound to a
secondary data source? Can anyone suggest the best way? I've been
spinning my wheels on this.

What I have is a simple SharePoint List, two colums, four rows. I
create a secondary data source and am able to bind the dropdown menu to
one of the colums. All is well except that I want to have "Select
one..." option that appears first in the dropdown. I don't want this
value in my data source.

There isn't a way to do this via a Rule. I can set the field value when
the Condition is blank line, but this is problematic. When a user
selects something else, the "Select One..." option disappears.

I thought about adding an element to the secondary data source in the
forms Load event handler but cannot figure out how to do that. And
maybe that's not the best approach or won't even work.

Related to this is, Where does the secondary data source reside when
the form loads? I would like to be able to see the actual XML data. Is
this possible? It would help me understand what's going on behind the
scenes.

Patrick