Just tried it and it worked with the VPN method!
1- Got 12 Months Xbox Gold (Europe) from G2A & 1 Month Game Pass Ultimate (Europe)
2- Used a VPN to change my region to Europe, then went to Microsoft redeem (not Xbox redeem).
3- It asked me to enter a code for redeeming in (my region) instead of Europe, I just hit next, and then it redeemed successfully. (I tried to redeem without VPN but it failed). My region is the Middle East, and I was on xbox.com/en-US when I redeemed the code, but my VPN was in Germany. So it doesn’t matter what website/region you’re on, all that matters is what VPN you use to enter the redeem website.
4- I redeemed the Game Pass Ultimate and it asked me to convert at a “conversion rate”, I hit yes.
5- It asked me to turn on recurrent billing to get 1 month free, I hit yes/
Now I have 14 months of GPU For 45$!
EDIT: This guide is to convert 12 Months of Xbox Gold into 12 months of ULTIMATE PASS.
How To Get Xbox Game Pass | Ultimate For 3 Years (Not sure if it Works)
1- Register a new account on Microsoft.
2- Connect your new account to Xbox, in region settings put “Argentina”
3- Now you have to buy 12 keys of GamePass Pc 3 Month Trial.
4- Open 12 tabs in your browser and go to: https://redeem.microsoft.com/
5- Put your keys inside of the field BUT DON’T PRESS CONFIRM.
6- After filling all 12 tabs with all your 12 keys, go to each tab and press CONFIRM in all 12 tabs as fast as you can.
7- Look how many keys got activated, if all 12, buy Game Pass Ultimate from Argentina for 39 pesos and then you will get 36 months of game pass.
If you got 6 or more activated keys, buy 1 year of EA Play and 1 month of Game Pass Ultimate, then you will get 35 months.