This is a fairly easy process depending on where you are intending to add the template. These type of templates are called ‘Site Admin Templates’.The template is going to be part of an existing site collection take the following steps in the existing SharePoint site.
Once this is complete you can use the template for your new sub-sites with the existing SharePoint sites. The will be listed under the ‘Custom’ tab of the ‘Template Selection’ in the ‘New SharePoint Site’ page. You can find a great selection of Microsoft templates here Application Templates for Windows SharePoint Services 3.0.