Since v2.2.1 you can add custom-defined fields to the order form.
Setting up the custom fields
The custom fields can be entered if you login as admin and then go to Site Setting. There go to the "Setup" tab and on the bottom, you will see the "Custom fields" section.
The custom fields are defined using JSON syntax. So make sure you have a valid JSON string.
Here is an example of custom fields. You can use this tool to create the JSON.