"If they don't have a visa, they will not come to Tanzania"
Translation:Wasipokuwa na viza hawatakuja Tanzania
No, 'to have' is 'kuwa na' or more directly translated, 'to be with'. Otherwise, 'hawana' means' they don't have', so as you've said, the alternative would be 'Kama hawana viza hawatakuja Tanzania'
The affirmative of 'hawana' is 'wana', which is shortened from 'wako na', a present tense conjugation of 'kuwa na'