Hi polevik,
You can indeed do as you are asking
BUT there are certain things you have to do to avoid making the device an expensive paperweight!
Firstly make a
backup of any files on the device - get them off to your PC as any ROM flash is like a hard reset and
WILL wipe the device.
HardSPL
IS a must if you want to flash ROMs from different manufacturers and carriers.
All flashing activity must be done with the SIM Card and Storage Card out of the device to avoid problems.
Once you have flashed HardSPL onto the device you could then proceed to flash the ROM. O2 are due to release a WM6.1 ROM for the Orbit 2 in a couple of months, most people can't wait however - me included!
There are ROMs from HTC and then all the 'cooked' ROMs from XDA Developers' website.
My personal choice is udK's Syrius ROM for Polaris - it has the new (Diamond-based) TouchFlo system (2D, not 3D) and works well and looks stunning.
Only do this if you are confident in what you are doing.
Never flash a device through a USB hub - always direct to a port on the PC