Hi Qiang Fu,
You can set up two number ranges, internal and external, and while creating automatically in the process SU will obtain a number from first number range, and while creating manually it can be in number range 90000....0000 - 9999...9999.
If you don't want to mess with customizing, maybe there is some kind of indicator which you can lookup in t-code LS33 for your SU that you can use to logically distinguish between auto & manual.
Regards,
Mihailo