its pretty hard moving from one software to another, i think it would be an amazing feature that could save a lot of time to a lot of people !
Afaik you can’t really effectively generate pixel art normal maps, at least not the way you usually generate them from height information, without losing data / blurring it because the only way to generate from height is to compare adjacent pixels and store their difference in both. Or you mean the kind of generator that kinda guesses it based on sprite outline. Neither really a focus of pixel art software.
Good pixel normal map painting would depend on having good tools to paint with, like a color wheel that works on normal angles rather than colors, and transparency and blend modes to mix colors in normalized vector space so the B channel always has correct data also.
A naive height to normal filter, normal color wheel and previewing could be implemented pretty easily I guess even as plugins, but they wouldn’t make Aseprite a very good tool for drawing pixel art normal maps unless there were some proper tools to go with them.