Drat!

Just gave this an hour of my life and have yet to find any key combination to toggle the insert mode.
It must exist.
I'm in insert mode by default and can't find a toggle to overwrite mode.

About to try some mouse tricks.

Nada!
Found some interesting toggles but no insert/overwrite.

I give up.
You might just have to compose in Word, wordpad, or your favorite editor, then copy n paste.

Best of luck with it.

Weeze