I streamed how I imported the lego head (https://mxb-mods.com/lego-head) couple days ago.
Maybe this might be helpful if you're interested in importing a helmet.
It's same procedure as anything else, just the naming of the parent pivot is important.
Take a look here:
https://www.twitch.tv/videos/912901216
Following what I do step-by-step is very important, you see more than you need to, though.
Good luck :)