This has always been possible in new-formats. Put the files in your pack's folder, using the same filename as in gfx/menu or gfx/panel (as applicable).
For high-resolution (panel graphics only), add "-hr" to the end of the filename (before the ".png" extension). High-resolution, of course, requires NeoLemmix V12.8.0 (currently only available as an RC build).
Known bug (that was discovered while double-checking that this works): If a pack provides custom low-res graphics but doesn't provide high-res ones, NeoLemmix falls back to default instead of upscaling. (EDIT: Now fixed as of commit 8e5717a.)