1. You need to make sure you're licensed to do it, you'll need a Sametime extranet license
2. Sametime Proxy is the best server product for enabling this. You could place your proxy server in the extranet, and keep your other sametime servers internal.
Our company, and I know of others, have done this kind of work for customers, providing simple links on pages using the Sametime Proxy toolkit, or taking it even further with hunt group type functionality where it finds the next user and outside users are just clicking with a "Help desk" but the back end systems take them to the most suitable userfor example.
Here are the slides I presented at IBM Connect 2013, which gives a simple example for suppor rep intetration on a page.