sorry for the long wait, I only now found your topic.
(Actually... considering that this is a HÂRN Forum, 3 days must feel lightning-fast...)
I don't have a full english document with a HârnMaster conversion of the Gemini magic system.
Back in the days, I translated the whole Gemini magic system into German, as I wanted to play it with a friend whose English wasn't very good.
Later, I used this magic-chapter for a HârnMaster house rule.
Basically, I changed all the relevant information to fit the HârnMaster system, including a passage on how to become a Shek-Pvar with the Gemini Magic System. This passage was basically taken from HârnMaster Magic, and tweaked to fit the Gemini system.
I gave all the relevant system information in the thread you linked.
In my German document, I also listed a couple of rules that should be included in the "Hârn Gemini Magic System" by taking them over from HârnMaster Magic, as Gemini doesn't have rules for this:
1.) Spell Fatigue
3.) Spell Research/Learning
To create your own, English-language "Hârn Gemini Magic System" document, maybe jus follow the same steps that I followed back in the days:
1.) Use the original Gemini text as a basis.
2.) Replace/amend all system-relevant information (use the info I have given in 2007).
3.) Expand the text by giving it a new first chapter, which is basically a conversion from HMM on how to become a Shek-Pvar. I gave the info I used. (This and some forword used up 3 pages in my document.)
4.) Bring everything in the usual Hârn layout - or any other layout you prefer.
That's basically it!
Of course, you will have to thing about how you handle things like damage.
You can keep the rule that each level deals 2 points of damage - but think about how the HMM system usually gives the caster some special effects if he rolls a CS (damage higher, etc.).
One way might be to treat magic attacks a bit like missile attacks, and the defender either ignores, dodges, or "dodges" with his Will x5 to resist magic (as described in Gemini).
I hope this helps!