I'll just add the idea of a plugin that wraps pandoc. Converting file formats, including .rtf and .docx and .odt, is pandocs core feature (it converts from "almost anything" to "almost anything").
--> see Any suggestions on what plugins could be created? - #23 by philip-n and Any suggestions on what plugins could be created? - #70 by tristinDLC
That said, you can already use pandoc manually as a standalone-tool: just feed it some markdown (e.g. copy and pasted from Joplin) and specify your desired output format 