Can I use a yoghurt maker for keeping my kefir warm?

No, yoghurt makers are designed to maintain a temperature close to 40 deg C, which is too hot for the Kefir Culture.

During the summer months in most of Australia just leaving the kefir to ferment on the kitchen counter will be sufficient.

In the colder months, a fermentation/aquarium heater is the most economical option. These can be seen here  - https://www.greenlivingaustralia.com.au/fermentation-heater-aquarium-heater.