Workspaces created by setting up a recurring meeting series normally
fall within the regular pattern you define. You can't go to your
Events list, tell SharePoint to create a new event and link it to
your existing series. (This is possible only if you start with a
single meeting and add individual meetings one at a time to your
series.)

However, I recently discovered you CAN provide workspaces for extra
ad hoc meetings within your series. To do this:

Click "Modify This Workspace", "Show All Lists", "Events", and
create an Events list with an appropriate name (such as "Additional
Meetings") and specify that it should use Series Items. When the
list appears on the front page of your meeting series workspace,
enter a new Event in that list for your ad hoc meeting. Indicate
that you want to create a workspace for it.

This actually creates a workspace subsite under your meeting series
workspace site, even though Microsoft's own documentation seems to
say this can't be done. (See http://office.microsoft.com/en-
us/assistance/HA011611351033.aspx)

If you modify the view of your new Events list that is displayed in
the front page Web Part, you can easily expose the link(s) to the
additional workspace(s). Your newly-added meeting(s) won't be listed
with the others in the nav bar at the left, but it will be clearly
visible in the Web Part, described as exactly what it is: an
additional meeting.