OK, I found an easy way to add a storage driver.

The driver I'm adding is Compaq Smart Array 5.x
The Windows .NET/XP driver files from compaq are:

CPQCISSM.CAT
CPQCISSM.DLL
CPQCISSM.INF
CPQCISSM.SYS
TXTSETUP.OEM

Create a directory for these driver, I'm using "I386\system32\drivers\cpqcissm\"
Copy *all* the driver files into the "cpqcissm" folder.

Edit the file "i386\system32\winpeoem.sif"
Change 3 lines:

[OemDriverParams]
OemDriverRoot= Drivers
OemDriverDirs= cpqcissm

Note that you can add a second directory to "OemDriverDirs", for example OemDriverDirs= cpqcissm, mydriver2

I've tested it on a ERD commander 2002 bootable CD and it worked!

Regards,
Bart.