Short Answer: yes.
Long Answer: Yes, but…a full answer to this question includes a technical answer and a practical concern.
First, the technical answer
You can install a .pkg or .mpkg using this syntax:
sudo installer -verboseR -pkg "/path/to/pkg/foo.mpkg"
If the installer isn't 'signed' properly, you'll need to add
sudo installer -allowUntrusted -verboseR -pkg "/path/to/pkg/foo.mpkg"
You may also need to specify where you want it installed, using
-target / (I'm not 100% certain this is required, but it's a good idea):
sudo installer -allowUntrusted -verboseR -pkg "/path/to/pkg/foo.mpkg" -target /
Now, the problem is that
sudo is going to ask you for your administrator password when you try to run
installer. If you want to automate this, you need to tell your Mac not to require your
sudo password when running the installer. To do that, you can add this line to your
%admin ALL=NOPASSWD: /usr/sbin/installer
man visudo for instructions on editing that file.
Second, the practical concern
If you are the only person who uses your Mac, adding the above line to
/etc/sudoers is not a big deal.
However, if this is a shared Mac, then other people who are in the 'admin' group will be able to run
/usr/sbin/installer without being prompted for their password.
Also, obviously if someone gets into your 'admin' account, they too could, theoretically, cause mischief with
/usr/sbin/installer. Although I am at a loss to think of exactly what they would do, it's a trade-off of security versus convenience.
Third, a github script
I wrote pkginstall.sh to do some nice things like log the process, as well as tell you whether or not you are supposed to reboot after installing the package.
Last but not least: Automate "how?"
As far as how you want to automate the installation, that depends on more specifics of what you are trying to do. You could, for example, make a folder such as ~/Action/AutoInstallPKG/ and tell
launchd to install any .pkg or .mpkg files that are added to that folder, and then move it aside afterwards.
I have been meaning to do something like this for a long time, and so I finally put it together. You can find it at https://github.com/tjluoma/autopkginstall. Installation instructions are included at Github, so I won't repeat them here.