A "merge" statement might also work for you i.e.
Merge INTO EquitLst E
Using (SELECT :eqsymbo as eqsymbol, :eqname as eqname FROM rdb$database) D
On D.eqsymbol = E.eqsymbol
When not matched then insert (eqsymbol, eqname) Values (D.eqsymbol, D.eqname);
although in the select statement you may have to cast the columns to the same type as the columns to be inserted. However, a simpler solution could be to use "EXECUTE BLOCK" with your parameterised values "declared" as variables, as this would allow you to use PSQL as an inline query.