Can you set the fallback= of a field

I would like to know if it is possible to set the fallback of a custom field:

[country,fallback=UK]

What I mean is, I think that it might be a good option to be able to set the fallback= at the time that the custom field is created so that from that time onwards the fallback will be used if none is supplied in the HTML when email are written.

It's kind of like setting a default at the time you create the custom field.