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