I could not find the way to move non-elastic pool dbs to an elastic pool db instance.
alternatively , I took the backup and 'export data-tier application' and restored that onto elastic pool as 'import data-tier applicaiton' then configured that db under elastic pool to be listed as elastic pool
so complicated Microsoft made it, in reality it should be allowing to switch add any db in Azure to add to elastic pool.
rember I needed the down time for the db to perform this move. kind of migrating the db from one server to another server involves all those tast similar to database migration.