Once a template is in SmartLETTERS it is actually a pure HTML/CSS template with locally stored images. If you have the requisite skills you can edit the raw HTML by editing the template and clicking the 'View' > 'Source code' menu item. There are advantages to this approach in that you can adjust the template in any way supported by the HTML2PDF library including adding headers, footers and page breaks as well as adjusting margins.

Perhaps the simplest change to make is to adjust the margins of your template. You will see at the top of the HTML source code the line:

Just adjust the 72pt values accordingly. They specify the margins in the order top right bottom left so adjusting the third one to 30pt would adjust the bottom margin to 30pt.

You can add a header or footer to your content by adding the below to your HTML. It does not matter if this is at the end or start of the HTML although this effects where the code appears in our normal editor. The below are examples for which you can adjust the content in the normal editor after you have added them to the source code. The page header here has a 72pt margin applied to it and 20pt at top. This is so the content is centered nicely although you can remove the DIV and /DIV to remove this margin.

and

you would then have the normal content with it's surrounding margin DIV too i.e.