Is it possible to use SELECT in COMPUTED columns?
Yes. You need to use two pairs of parenthesis. Example:
ALTER TABLE t1 ADD relation_count
COMPUTED BY ( (select count(*) from rdb$relations) );
Please note that COMPUTED BY expressions that refer to other tables are not very good in performance as new select is run for each row. It is much better to create a view or a stored procedure - or use EXECUTE BLOCK if you wish to do it in a single statement.