No SLES11 SP3, o comando usermod
(do pwdutils 3.2.15) suporta uma opção -A
que fará o que você deseja:
-A, --add-to-group group,...
With this option a list of groups can be specified, which the user should
become a member of. Each group is separated from the next one only by a comma,
without whitespace.
No SLES12 e no OpenSUSE 13.1, o comando usermod
(do shadow-utils mais usado 4.1.5.1) suporta uma opção -a
a ser usada em conjunto com a opção -G
:
-a, --append
Add the user to the supplemental group(s). Use only with -G option.
-G, --groups GROUP1[,GROUP2,...[,GROUPN]]]
A list of supplementary groups which the user is also a member of. Each group
is separated from the next by a comma, with no intervening whitespace.
If the user is currently a member of a group which is not listed, the user
will be removed from the group. This behaviour can be changed via the -a option,
which appends the user to the current supplementary group list.