I don't recommend what you're proposing because you won't get a stable reading on the sight while draining and you'll also have to plug the top of the sight opening if you want to siphon out to the very bottom of the vessel. You're better off adding new hole and fitting for the sight glass
If despite those two weaknesses you still want to do it, you'll need an NPT to compression fitting (very common). I'd recommend high temp polycarbonate available at mcmaster.com for the tubing.
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