XF 2.0 Expandable dropdown list for custom field, how?

[Jimmy Vu]

How can I create thread custome fields which allow drop down list in hierarchical format

For example:
Custom field Choose car": Audi ->then there is another dropdown list of different Audi versions. If Mercedez is chosen then the sibbling list will show Mercedez car brands

Same for choosing city names with different districts in them

Thanks

Jimmy