how can i added minutely cron to run the importer for ex. every 20 minutes ?
its more effective to run it with few ads many times than running it one time every hour with 100 ad

Run minutely cron once per 30 minutes and change processing function to minutely cron.

i added to importer/index.php

function imp_run_cron_minutely() { return imp_run_cron('minutely'); }
osc_add_hook('cron_minutely', 'imp_run_cron_minutely');

and changed the oc_t_imp_import.s_cron to minutely

am i missing anything ?

Looks correct