Select a literal value for INTO ... FROM

I’m trying to do a rollup from two sources into a single table. The first source has the tag value I need already as a field, but the second table does not. I’d like to be able to use a string literal for the value of one of my tags. Something like “select time, value, state, ‘external’ AS source FROM xxx INTO yyy”. This would write entries into yyy with the value for source as ‘external’. Is something like this possible?