The topic posted by OP is different from your question and I presume, you have not gone through that. Once again, please go through that and also my reply. It is about Billing Plan Number Range and not Billing document number range.
Nevertheless, to answer your query, yes it is not possible to have external number range for billing documents. In fact, if you check in VOFA and select any document type, you cannot see External number range field; though a field is provided for this in table TVFK. SAP has provided two user exits which are
- USEREXIT_NUMBER_RANGE (Module pool SAPLV60A, program RV60AFZZ)
- USEREXIT_NUMBER_RANGE_INV_DATE (Module pool SAPLV60A, program RV60AFZC)
The first one is used for number range exclusively and the second one is to have a different billing date. With these user exits, you can fetch different number ranges; for example Sales Organization wise or Plant wise but it should only be internal.
G. Lakshmipathi