De man join
-a FILENUM
also print unpairable lines from file FILENUM, where FILENUM is
1 or 2, corresponding to FILE1 or FILE2
-e EMPTY
replace missing input fields with EMPTY
então
join -a1 -e 'NA' -o 0,2.2,1.2,1.3,1.4 A B
1 A n m n
2 NA m n m
3 B n m n
4 NA m n m
5 C n m n