Ruger Mark III 22/45 barrel threading

[TKM]

I remember the first time I posted what could be considered a felony on Calguns.net....

Nope, must have been thinking of something else.

[NATEWA]

https://infiniteproductsmidwest.com/new-this-week/MK3-1228-BS/

[ar15barrels]

All this trouble just to get a off roster Ruger .22lr?

Yep.

[ar15barrels]

Assuming a guy was moving to California from a free State, and assuming he wanted to bring along his Ruger MK3 22/45 lite with threaded barrel. How would he or she make the pistol a "fixed magazine"? How would you load it?

Have to look into how the magazine latch moves and figure out how to pin it into the gun.
You would take the top end off the grip frame to access the magazine.
That would most likely require you to upgrade to a MKV before moving into CA if you actually wanted to keep the threaded muzzle because I don't think fixing the mag into a MK3 is practical.
For that matter, perm attaching a muzzle device or thread protector is the most practical way to make them legal in CA so just do that before you move into CA.