Is there any way to put in Automic Postconditions a restart in seconds and not just in minutes?
No. In the Postconditions you can only specify minutes at RESTART TASK.
However, you could try the following: In the Postconditions use RESTART TASK with 0 minutes and put the following in the pre-script of the job:
:IF SYS_ACT_RESTART = "Y"
! WAIT 30 seconds
: WAIT 30