If you already have a rooted desire, you can try the steps that mercianary provided. I am not sure, but maybe you don't need s-off because you will flash an "official rom".
Try these two steps
dd if=/dev/mtd/mtd2 of=/sdcard/unroot/boot.img bs=4096
dd if=/dev/mtd/mtd3 of=/sdcard/unroot/system.img bs=4096
And then without s-off try to push them with fastboot, you will get an error if it doesn't work, but then it hasn't done anything, so you don't need to worry.
Do all this on your already rooted desire of course. No harm done if it does not work.
If that works, you can try a temproot on your non rooted desire. Then do the same things from adb, then do the steps from fastboot on your already rooted desire (but don't push the recovery yet).
Samsung Galaxy S 4 I9505 | WanamLite v2.6
Samsung Galaxy S II I9100 : AOKP 4.2.1
HTC Touch Pro2 | ROM: Valkyrie 6.5.23115TR2.5
HTC HD2 | DarkStone1337 [FroyoStone Sense - V3.2]
HTC Desire : Oxygen v2.1.3