Autonumbering Account for select customers

We have multiple source systems importing customers to R12 and currently the System Option for autonumbering is deselected because those source systems have their own customer number. We simply store that number

Now we are acquiring new source systems which do NOT store a customer number.

So we need to be able to auto-create a Customer Account number for a select group.

I have not found anything on MOS that describes how this can be done without a customization.