This is what the Janome website says:
It's Easy To Customize Built-In Stitches
Go beyond adjusting the length and width of the built-in stitches. You can bring any decorative stitch from your MC12000 into Stitch Composer and customize it any way you like. Then you can save your custom version as a .stx file--don't worry, the MC12000 won't allow you to save over an original built-in stitch.
But how can I bring the stitch from my MC12000 into Stitch Composer?