For this, you don't need any tool except for LNZPro, preferably version 3.2 or later.
You can download it from my "Nicholas' Tools" pages.
First off, choose a copy of the breedfile that you want to use to make your own.
Open it into LNZPro and navigate via the little crosses until you can click on and highlight
the 2-letter item under LNZ. In this example, the Black and White shorthair cat,
that 2-letter item is BW
Next, open into LNZPro also the pet from which you want to make a breed and
similarly navigate until Adult is highlighted
Click on the right-hand pane, where you see [Texture List] in the above example.
Now choose "Select all" from the drop-down Edit menu
Choose "Copy" from the drop-down Edit menu
Click on the right-hand pane beside BW, just below bwkit.lnz,
and select from there to the end of the pane, scrolling down with the scrollbar at the side until
you get to the bottom of it.
Choose "Paste" from the drop-down Edit menu.
Ta-da! The adult part of the breed is now done.
Go through the same process of selecting and copying the Child LNZ of the pet,
and in BWKIT (or the equivalent kit or pup LNZ in your breedfile) select from
just below \ptzfiles\cat\bw\bw.lnz (or the equivalent in your breedfile) to the bottom
of that pane.
Choose Paste, and Save.
Your breed is now completed, if you wanted it to be an overwriting breed.
Go on to page 2, here, if you want it to be a non-overwriting breed