If you have any randon free XML editor(such as Notepad++), you can do it freely.
Open your MekHQ save file that doesn't have any custom rank with a XML editor, and on line 7 to 10 you can see this.
<rankSystem>
<!-- Star League -->
<system>0</system>
</rankSystem>
That is the default, for the default rank is Star League rank.
And also open your MekHQ save file THAT have the custom rank what you have set. and copy all the area that <rankSystem> to </rankSystem>, paste it to your new MekHQ save file, then save it. if you did that, it's done. Even if your force have some personnel with ranks, if they have correspond rank already then they will get the new rank because it replaces the former one.
Still, I do want that save/load custom rank and some quality of life changes. It was always a headache for me. Especially for keep typing --MW part.... If I knew I can do it by XML I was already did that.
Besides, I know the way to do just now. I didn't aware that it saves the rank top of the file.