I don't think that you can create sprite sheets in Pixelmator. At least I haven't seen any mention of it. However, you might want to check out Pencil or Pixen (both free).
I've used Pixen…I'm not very good with pixel art. I'm better with just doing freehand. Pencil is ok, but Pixelmator is nicely done close to Photoshop. I guess I will have to figure a way to keep copying images and making slight changes.
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