Look in the template for something that looks like "$template_hook[somename]". If it is located where you want to add some text, great. If not, you can input something into the template in that format "$template_hook[yourname]". Then, create a template, your_template, and put in the contents you want to enter there. Then, create a plugin that uses a hook on that page and in that plugin just put
PHP Code:
eval('$template_hook[yourname] .= "' . fetch_template('your_template') . '";');
(Replace "yourname" with "somename" if you are using an existing template hook.) This will insert the contents of your template, your_template, at the place you put the template_hook, yourname.
(I could have sworn I saw a good article on this, but right now all I'm finding is this:
Template Hook )