It will be nice if we can do something like this:

  merge into fbl_row t
  using (select :vdid vdid, :vpid vpid, :vqty vqty, :vcost vcost from rdb$database ) s
  on (t.did=s.vdid and t.pid=s.vpid)
  when matched then
      update set pqty = pqty + s.vqty, rcnt = rcnt+1
  when NOT matched then insert (id, did, pid, pqty, pcost)
      values(gen_id(gen_test2,1), s.vdid, s.vpid, s.vqty, s.vcost);
  returning id
  into :vrow;

As far as I understand current version of FB does not support it ? (http://firebirdsql.org/rlsnotesh/rlsnotes210.html#rnfb210-merge)