dopuniti

1. to supplement; to complete
   Molim vas da dopunite spisak. — Please complete the list.
   Dopunio je svoj odgovor novim primerima. — He added new examples to his answer.
2. (of a glass, tank, balance) to top up; to refill
   Konobar nam je dopunio čaše. — The waiter topped up our glasses.
   Moram da dopunim kredit na telefonu. — I need to top up my phone credit.

The imperfective is dopunjavati; a phone top-up is called dopuna.