I feel stupid asking this, but as i said im not very experienced in Maya. Could you please clarify what you mean? How do I convert something to a shell?

There was some UV objects that were not fully selected when i used the "Move UV Shell" tool. I tried Polygons > Merge UV's and it worked. I could then use layout for that texture editor file. I can't find the defect shell on a different texture though.

Edit: I just worked a bit with it. It can use it right now. It cleans it up weirdly though.
What are your thoughts on this: https://imgur.com/a/TiNd2 ? (top picture is the layout i made. Bottom one is Auto layout.)