Certainly, you can thread the steerer if if it meets certain conditions, and you have the tools and skill.
The first is that it needs to be made of steel, and not be too hard for threading. You can test for this with a file. The second requirement is that it must have a 1/16" wall thickness, as in 1" OD, with 7/8".
Assuming the conditions are met, I suggest letting a skilled shop do the job, since the cost of decent tools for this is significant.
If you decide to have it threaded and the LBS refuses or wants too much, PM me & I'll give you a referral.
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