Can I have a SQL statement in a Custom Function in Crystal Reports 2008

I'd like to create a custom Crystal Reports function which does a database lookup. Is this possible? In the pseudo code below "v_le_afe_xref" is a dataource in the report. A syntax check fails on the select.

Function legal_entity_co (old_co AS number) AS number

SELECT v_le_afe_xref.NEW_BURKS FROM v_le_afe_xref WHERE old_co = v_le_afe_xref.OLD_BURKS

END FUNCTION

Thanks.