The system does not suggest automatically numbers for external number ranges.
This customer requirement looks more like a wish than a real business need.
You can try with an additional development of course, but it seems like a real waste of development effort. Especially that there is no legal requirement for customer master numbering in any country that I know of.
If a person is unable to remember to enter 3 characters + 3 digits when creating customers manually (especially considering that the system shows a message with suggestion of the number range if you enter incorrect data), you probably should not give such responsibility to him...seriously... who knows what mess he will cause when maintaining taxes/payment terms/reconciliation accounts/tax numbers 
.