It is possible on Odoo Online. You probably would want to create a sequence pattern, create a new field on the customer form (or re-use the internal reference like you did), and have a server action that fills it in automatically based on the sequence.

If you want Odoo to do it for you, it can be done with a success pack: https://www.odoo.com/pricing-packs

But why do you need to keep those numbers if they are specific to your previous software? Do you need to associate those specific numbers with something else in Odoo?

For Odoo online you don't have many options, just ask Odoo S.A for support about that here

https://www.odoo.com/help