CM has a Form (XDP template) as well as a Letter (Correspondence Template that uses a Form) - Not sure which one you are looking for when you said “XDP letter template” [but my guess is that it’s the former that you may be looking for].
So, if you are looking to add an XDP template, i.e., create a Form, you may use the FormService APIs (createForm(…)) to do so. And in case you are looking to create a Letter you should be using the LetterService APIs (createLetter(…)).
These APIs (available in the adobe-acm-client.jar in your installation’s client libs) are exposed as Spring beans, as part of the Asset Composer building block.
(you can easily expose a Spring bean as a Web Service - http://static.springsource.org/spring/docs/2.0.x/reference/remoting.html)
You can go through java docs