Okay here it goes, its very similar to the HTC method which requires alot of tinkering.
Firstly, you need to be Dev Unlocked. Search for Chevron Unlocker if you dont already have it.
Secondly, you will need Tom XAP Installer which can be found as a sticky at the top of the thread.
Lastly you will need nico's Registry Viewer XAP Package which can be found here: http://forum.xda-developers.com/show...1&postcount=19
Now when you have all the above done, Load up Registry Viewer and goto:
You will see some numbered subkeys. You can edit any of these but I reccommend editing .
Now navigate to  and tap FileName this is where you enter the desired xml.
Now I have a UK Omnia 7 but I flashed the Unbranded XENJK1 ROM which is for Netherlands. So whenever you run the Network Profile App you will see the Network for the sim card you are currently using plus 4 other networks to choose from which will be Netherlands Phone Operators.
Here is a little example to run your desired provxml file. Remember we tapped on FileName in the  subkey, now clear the current filename thats already there and type in: UK_XEU_BASE.xml
Exit Registry Viewer and startup the Network Profile App and Tap on the Top Profile. (This is why we edited ). You will see the loading dots in the middle of the screen to tell you the device is busy (thats the provxml installing). When its done, exit the Network Profile App and restart it. You will now notice that ALL the UK network providers are now in the Profile list.
So this definately works, we just now need a way to create our own provxml files and copy them to the Windows folder on our Omnia 7!