Add custom fields to contacts (e.g. a customer/member ID)

I need to store extra information on a contact that isn't in SAGA's built-in fields — for example a customer or member ID from another system.

I'd like to create my own fields, fill them by hand or from a form, and lock them so the rest of my team can't edit them afterwards.

Right now the only option is to squeeze this into the description, which isn't reliable or filterable.