I dont know about SCCM (were too poor to buy it) but this is the command I use to use a config.xml for an XP to XP migration
scanstate %EXPORTDIR%\%computername%_%user%\ /config:%USMTDIR%\config.xml /i:%USMTDIR%\miguser.xml /i:%USMTDIR%\migapp.xml /v:13 /l:%EXPORTDIR%\%computername%_%user%.log /targetxp /localonly /ue:*\* /ui:domain\%USER%
I have an heavily edited migapp.xml, Im a bit dubious about creating a customapp.xml and integrating it with config.xml even if it is best practice, introducting errors into the xml is too trivial
Generating the config.xml is:
scanstate /genconfig:config.xml /i:...