How to define challenge password (SCEP) manually in windows 2008 Enterprise CA

reference doc (I can't past link, so I just list doc name):
Network Device Enrollment Service (NDES) in Active Directory Certificate Services (AD CS)

==

The doc said this one-time password is random.

We can modify Registry to change password length and valid time.

But I can't find how to define this password manually.

e.g.  I want to set 3 password in password list/cache : aaaaa, bbbb, cccc.

Someone know how to does this ?   

Thanks.