According to http://support.microsoft.com/kb/825430 adding additional words or modifying the dictionary is not supported.
I found a 3rd party app that can make it happen here:
Provides server-based spell checking for OWA 5.5 and 2000
Extends OWA 2003 server-based spell checking capabilities
to both PC and Mac platforms,IE 6 and non-IE6 browsers.
Offers custom dictionary capabilities and no client-side installation. Supports multiple languages and automatic “Check on Send”.
TechGenix Reviews & Awards
- Company: ISOTA
- URL: www.owaspellchecker.com
- Email: firstname.lastname@example.org
- Price: from $399
or this one that offers a free trial: