Originally Posted by veole
I tried Mobipocket creator, but it does now allow to import already made mobi files.
I know that I could use Calibre to convert from mobi to epub, then edit the book in Sigil.
I'd like to know if there is a mobi editor or a better solution.
In short, No.
Every time you make edits to the source for a mobi, you have to rebuild it. One edit; one hundred edits, doesn't matter. You rebuild it. That's why most of us make the ePUBs first, make all the various edits, etc., therein, and then build the finalized ePUB into a mobi file. If you're determined to do mobi only, then what AnemicOak said is the only way: unpack an existing mobi with mobiunpack, edit the bits you want with an html editor, and then rebuild the mobi using KindleGen or KindlePreviewer.