1) You all will be surprised but you need to get Wizard's Shell tool, which is being used to flash a G4 Wizard.
2) Get hold of the .nb file and rename it to 80040000-OS.nb and keep it in a seperate folder(Not in that shell tool folder)
3) Now Active Sync yr artemis.
4) Take any nk.nbf of that Wizard and place it in the Shell tool folder and execute the shell.exe
5)Let the shell tool get the ROM info,Extract OS and give the message "Done".Wait here, do not press OK in the shell tool window.
6) Now minimise the shell tool window, delete the 8004000-OS.nb file from the shell tool folder and copy here the earlier renamed wm6(80040000-OS.nb) to the shell tool folder and now press the OK button, which will start flashing nad show this info in the shell tool window.
7) Contrary to the Wizard flashing, this shell tool will not inform you that flashing has ended, so u have to keep the process ON for 50 mins (for a safe side)
8) Disconnect and do a Hard Boot and YES u hv got it..
As a safe practice keep the T Mob initial ROM ready..in case u miss, you can reflash the ROM and redo the things as mentioned.
I can tell u that, It tool me 3 attempts to get the WM6, but I hope with above steps, you can get in one go.
It is too late so attaching a few screen shots to build in the confidence