You can!!
Just make sure that you enable extended auth for remote access users but disable it for your pre shared keys already in use on your site to site tunnel(s).
Here is a great example of what you are trying to do:
<a href="http://www.cisco.com/en/US/tech/tk583/tk372/technologies_configuration_example09186a0080094685.shtml">Configuring IPsec Between Two Routers and a Cisco VPN Client</a>
Last Wiki Answer Submitted: June 17, 2010 12:55 pm by Matt Mather3,610 pts.
All Answer Wiki Contributors: Matt Mather3,610 pts.
If you live outside the United States, by submitting your email address you consent to having your personal data transferred to and processed in the United States.
sure can – Great link posted by Matmather too…