No, I don't think you can use them on unbound fields.
I use the DSUM and other D functions with the same queries that I use to generate my report to do such calculations.
DSum("[AMOUNT]","qry_comp_amt_sub","[COMP_ID] = '" & [COMP_ID] & "' AND [client_id] = " & [client_id])
The DSUM function...