In multi-source queries, is it feasible to specify the type of attributes to be returned?

Yes, you can use return, it is an arbitrary expression. So, whatever you want to return from the “from,” you can.

For example

define "Encounter with Assessment Result":

  from

    "Valid Encounter" Encounter,

    ["Assessment, Performed": "Specified Assessment"] Assessment

      where Assessment.result in "Specified Assessment Result Valueset"

        and Assessment.relevantDatetime during Encounter.relevantPeriod

      return Encounter