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I don't know where you got the idea that rle8s store the sprite masks but this is incorrect. Just ignore the rle8s, they're not important. To add a mask to your sprite, assuming you already have some sort of mask from the same place you got the original sprite from, you'll need to spin it with SpinApp again to get the 6x6 grid and then open the rleD for your sprite in EVNEW and choose import. Select the mask radio button and find your mask image. This will be imported into the same rleD as rleDs contain both the sprite and the mask in the same resource.
I see. Thank you.