To add SharePoint templates via command line you will need to run the following command. Note that the description is optional.
stsadm -o addtemplate -filename <template filename> -title <template title> [-description <template description>]
You will be prompted to do an iisreset to complete the template installation. Once iisreset is complete login to a web site create a new site and under “Custom” tab for “Template Selection” you should see your new template as an option.
On an odd side note these newly installed templates will not appear as expected in the template gallery but they will be there when you go and create new subsites under the custom tab.