You can't make it install suggestd automatically. However, you can get it to not remove them, until they're not needed by anything. From the aptitude manual:
Code:
Option: Aptitude::Keep-Suggests
Default: false
Description: If this option is true, aptitude will keep automatically installed packages on the system as long as any installed package suggests them.
So, if you put
Code:
Aptitude::Keep-Suggests "true";
into /etc/apt/apt.conf you will get this behavior.