When creating SCCM boot.wim what modifies the winpeshl.ini file?

I need to add a command to run before tsbootshell.exe in the winpeshl.ini. Other than cracking the boot wim and manually modifiying winpeshl.ini how can it get Configmgr to do this automaticlly? The post for putting it in the unattend does not work for me because tsbootshell.exe has already run at that point so the only place i can make this modification is in winpeshl.ini.

Thanks,

Uly