Been looking around for the same thing, the closest I came to was developing a Custom Component to achieve that. Currently, only following field types are supported:
Email, Phone, Picklist (not including dependent picklist at this time), Text.
There's a way to just add a checkbox field on the Pre-chat form global action layout after creating a checkbox on object, but it's not recommended at all since it will be overridden next time the Pre-chat form is edited.
IMPORTANT Create and edit your pre-chat forms from Embedded Service setup, not at global action. Editing global action can lead to unexpected results. Each time the pre-chat form is edited, the Embedded Service global action is recreated and prior updates to action details or predefined fields are reverted.
Embedded Service setup creates quick actions specifically for pre-chat. Using these quick actions for other purposes causes pre-chat setup to fail.
Please do mention if you were ever able to figure out a way to achieve it somehow without code.