Can I use IF in regular statements (not PSQL) ?

You cannot use IF, but you can use CASE instead (which is almost the same). Here's an example with employee database:

SELECT PO_NUMBER, ORDER_DATE, case when PAID = 'y' then 'PAID' else 'NOT PAID' end, TOTAL_VALUE
FROM SALES;

CASE is introduced in Firebird 1.5.

If you need more complex expressions that are not possible with CASE, and still need to do it in a single statement, you can use EXECUTE BLOCK with Firebird 2 and above.

Also, you can use IIF (Firebird 2). For example:

SELECT PO_NUMBER, ORDER_DATE, iif(PAID = 'y', 'PAID', 'NOT PAID'), TOTAL_VALUE
FROM SALES;