Did you use all 20 segments (impossible on an AONE3) and 4 segments in Europe? If you haven't, you still have segments left and this isn't really your last segment (just the last that you have booked). You cannot buy extra segments in Europe but if you started as CDG-HKG probably you haven't used 3 segments in Europe already. So you could book LHR-LIS-MAD-CDG if you have 3 segments left, or if you only have 2 you could even book LHR-LIS/MAD-CDG. Ticket is valid for 1 year from the date of your first flight CDG-HKG. Changing routing will cause a USD 75 change fee while changing the date should be no fee though BA will charge you a GBP 25 fee so you want to do this at AA in LHR not BA.