What you can do is you can load the .txt string into Heromachine 3 and when you load the body (assuming you have it saved) there is a little check box under the OK button that says “Keep currently loaded items?”. Just X that little guy out and when you load the body file, the helmet components you’ve already loaded should still be there.
Screenshot for reference (you can see the little check box X’ed out at the bottom):
