Exactly, in the current version you can't repack that firmware file, because it is in ROFX format...
And, I think I've already introduced the support for ROFX, but I need someone to test it! If you're available, then send me a pm and we will make a couple of test

so will be possibile to flash the E51, N95 and all the devices that have a ROFX image.
The next feature I will introduce in NokiaCooker is the ability to change the ROFX images...
I think I've already implemented a working solution, but I need someone to confirm that it correctly works...
So, who owns a model containing a ROFX image and wants to make a couple of tests to improve the software, please propose yourself on this thread writing your phone model (Note: make sure that your device supports the dead-usb flashing)
Unfortunately, I can't try on my N95 because it doesn't support the dead-usb flash, so, in case of error I will be forced to bring the n95 to the nokia center.