Hi @Galdred,
This sounds similar to another thread:
You can load the same file path as an image and as a palette, then use both to make an RGB color mode version.
I’m guessing you could also load as a Sprite, convert to RGB with a command, extract copies of the cel images you want, then close the sprite when done. I’ve not tried this approach, but I suspect the trick would be making sure that commands target the correct sprite and that no references to frames, cels, layers outlive the reference to the sprite to which they belong.
If you want to convert the composite RGB sprite to be indexed color mode afterward, and want to extract a cumulative palette, the “New Palette from Sprite” command is called ColorQuantization in the API. Check the source code, not the documentation, for parameter names.