If I understand what you're asking, this is already possible using the "Field Formatting" option in the preferences when creating/editing a custom field. If, whatever you place in this custom field should be hyperlinked (i.e. you add
http://www,invisionpower.com) and you want people to be able to click that link, then just add the following to "Field Formatting" and whatever web address is placed in the custom field will become hyperlinked - using your example, it would be something like:
<a href="{value}" TARGET="_blank">Discussion Topic</a>Hope that helps

Marko
**edit**
Just realised this was quite an old topic, so some may already know this!