Allow providing defaults for required fields having no column match

[smcoll]

i have multiple spreadsheets to import for one model. Each spreadsheet represents a different data source, and my model has a "source" attribute (which in this case is an FK) which needs to be set to a default value for the whole import. As it is, i can't import the file as-is without getting a "Source is required but has no match" error. i can modify the spreadsheet to get around this, but it's not ideal. It would be great if i could set the default for that field, even though there's no column in the spreadsheet.