Reducing the size of the opening in a wall.

What size header was used?
What's the loads above the opening?
Is it even a supporting wall?
Only need a bottom plate and 2, wall studs where you want the opening to end, there is no reason to fill the whole thing in.

Load bearing

They used an nominal 9" header. Looks like a 2x12 ripped to be exactly 9". It is load bearing - basement wall of one story townhouse in Nebraska.
My reading of the code says even if it were a full 2x10, I could only span 5'9" (I have a 31' wide house).

Got a picture?
Did they use 2, jack studs?
Is an inspector looking this over or is this something your coming up with?
If it was mine I'd just leave well enough alone.
I've installed many a 6' patio door with 2 X 10 headers and never had an issue.
A real 2 X 10 would have only been 1/2 wider.

Yes, there will be an inspector as this is part of a finishing a major part of the basement - adding a bedroom, bathroom, and family room.

Originally, there was just a door and framing so there was a post of two 2x4's in the middle. And, only one jack stud.

So, I'm adding one jack stud to each side of the 6' opening.

Actually, a 5'6" opening is just fine - maybe better. After thinking about it some more it does seem (as per your suggestion) that I should be able to add a bottom plate and two 2x4's six or so inches inside one end.

If the inspector doesn't like it I'm sure he'll tell me how to modify it to meet the code.

Thank you for your thoughts and advise.