you can add a new client install packages ( newer than your manager version or older one)
you are trying to add package to your SEPM
in that case you need to browse to sepm cd1 packages folder (newer or older version)
admin - servers- install packages
when creating the package uncheck single exe, select setup.exe, add it here
once done go to the group, there will be another add package option select that, use the drop down menu to add your custom one...
add it to the group