/usr/share/lxdoom/wad is owned by root so to use the filemanager you'd (probably) need to start that as root.

I'd use the command line if I were you (run as root):

cp /mnt/mmc1/some_file_name_here /usr/share/lxdoom/wad/

Will copy some_file_name to the same name in /usr/share/lxdoom/wad/

You may need to alter mmc1 to mmc2 depending on which SD card you're talking about.


Simon