I am continually amazed by the new features of NEXXT; they are simply fantastic! 🤩
Importing CHR from NES ROM: I apologize if my response is based on a misinterpretation of your words, but if you look closely at the screenshots, the graphic banks being ignored are not only sprites.
Additionally, it is quite common to find cases where the same CHR is used to store both background and sprite data.
Sonic NES | Castlevania 3 | Adventure Island 3
Lastly, I must clarify that I have not been successful in accessing the rest of the graphics with the procedure you proposed.
The format used by MSB: It looks promising, but I need to investigate further to understand it better, as I currently extract animations from the PRG ROM, thanks to the search tools provided by Mesen.