Adding a user-defined field to requirements artifacts

We are thinking about adding a field to requirements artifacts, either to a specific artifact type such as a feature, or to all of them.  For instance an RFC field to tie in with another unrelated system where users enter change requests.  We would lable the field as RFC and not make it required, but would want to be able to filter on it as needed in queries and views.  Is this possible?  We have recently upgrade to Release 4.0.