If you want to add a feature to an Office installation, use msiexec.exe. The command-line interface of the Windows Installer.
I dont have the possibility to try out this command line. Its just here for reference to give you a hint.
msiexec /i officefilename.msiORproductcode ADDLOCAL=myFeature /qn
This will add the feature myFeature to the office installation in silent mode. Remove /qn to show the dialogs.
To find out what the name of the feature is, I suggest you to analyze the log file (temp directory) after the manual installation or dig into MSDN/deploy guides for Office.
After figure out this command line, just write a batch file or a nice nsis script (to check return values, whatever).
Please let me know if it works.
Have a nice day!
MSIEXEC command line options