add a toilet?

You can by using a macerator type wc which pumps the waste through small bore pipework which in turn can be connected to the existing toilet stack pipe, but be prepared for the surveyour to tell you you have to put in a new section of 100mm bore stack pipe.
Check the macerator WC at your local plumbing shop some large DIY stores have them but they might not have anyone who can tell you how they work, so go to your local small plumbing shop first.

Seek advice from you building control office about the 'do's and don'ts' Sometimes if you phone at 9 you will catch a surveyour who hasn't left on his/her rounds of sites, and they will give you very helpful advice.
Bob

Bob
November 2005