Open your .lso template with the Environment you want to use. Leave it open, and then open the .mid as a song, NOT using import. When asked, leave current song open, and then when asked, use current song's Environment.
If you wanted to add the .mid to a song, do the same as above, except have the song open first, do the above, then copy-paste the existing song's material all over to the new song that was created when you opened the .mid as a new song.
Complicated, or PITA, but only way I knoe of to do this (currently).