You can use the DataExtension mechanism to add extra database fields to an existing DataObject:
class MyMemberExtension extends DataExtension
{
private static $db = [
'HairColour' => 'Varchar'
];
}
And apply the extension:
# File: mysite/_config/app.yml
Member:
extensions:
- MyMemberExtension
This will add HairColour as a field to Member objects.