I had no idea what a kumara was till I looked it up. Sweet potato.
OK - so it´s got plenty of starch, but the pumpkin has not. Yesterday I made some Indian sweet potato pakoras, which are pretty similar, and used besan (Indian chickpea) flour. That´s guaranteed to bind the mix.
I´ve never made the balls either, but off the top of my head, I´d say:
Grated kumara, grated pumpkin, finely diced (or grated) onion, diced chile peppers, pinch of nutmeg, tsp garam masala, s&p, besan flour to mix. Once you´ve got a firm mixture, roll them up into little balls (they don´t need to be perfect) and deep fry.