Hi Maria,
I don't think is possible, plus doesn't make sense, if one field have a constant, can not have a variable assigned (source field), if is a constant is a constant. 
As you probably already check, you can mark initial value in both cases, using a constant as a source of the value or using the structure as the source of the value.
In order to try to propose a workaround I need to know What do you really want to archive with this? I mean functionally speaking.
Cheers!
Luis