Not an OpenType expert but there seem to be two ways to store bitmap data: EBDT and sbits. From what I understand, both can store different sizes. For the latter the Microsoft documentation is pretty clear: "A font can also include different bitmap data for different sizes (“strikes”)"
Unfortunately, the bitmap font manipulation tools I'm using (monobit¹ and BitsNPicas²) do not support putting more than one strike into an OpenType wrapper. If there's some other tool that can do that assembly, I'd like to hear about it.
Also Glyphs 3 seems to support it. "Each glyph contains a single image file. However, images can be provided for different pixel sizes.". It's been years since I last worked with Glyphs and back then it was more of a creation tool than an editor. I don't know if that has changed.
https://learn.microsoft.com/en-us/typography/opentype/spec/s...