INSERT INTO + Selects - PostgreSQL 9.2
Fast and dirty:

 INSERT INTO dm.billables_links (billable_id,mobiuser_id) VALUES (
    (SELECT billable_id FROM dm.billables WHERE account_id = 32152 
    AND billable_id IN (select billable_id from junk.billable_id),
    (SELECT id FROM public.ja_mobiusers WHERE name_first LIKE 'Alisha%'));


or a JOIN, if an relation between "billable_id"(junk.billable_id) and "id"(public.ja_mobiusers) exists.