Your PP cropping can substitute the teleconverter ^^. Most people crop when shooting the moon anyway, even with the 100-400L.
Here's my humble effort with the 55-250IS. I don't like getting too close to the moon btw.
Larger size
here
This was taken on a tripod, but my dad has managed to handhold at 1/8 with IS, leaning against a parapet. IS = not a magic pill, but it really does do A LOT.
I feel like upgrading to the 70-200f/4L sometimes (for the superior corner sharpness and contrast), but I know I will miss the IS.