Christien, if you enable the debug menu in Safari, you can do it without an extension. However, it has almost never worked to allow me to view a site when they insist on MSIE -- I presume because of ActiveX content.
Or you can use PithHelmet or several other plugins to give it to you (all of which have some other primary purpose). Once you enable it, the option is called "User Agent," near the bottom of the debug menu.