No and no (however airlines soemtimes make mistakes so some people have reported having done these things; but the rules are unambiguous, it is not allowed). If you "omit" a segment that is a no-show for the flight and any subsequent segment reservations are often cancelled automatically by the computer. The released seats may not go back into the same fare bucket, making it impossible to reinstate the reservation on high demand routes. As for changing a transit into a stopover, that is the reason that ticket agents look so closely at your ticket -- there is an x next to the airport code. Stopovers require paying different taxes than transits, so expect to be scrutinized closely and to pay the USD 125 reissue fee, plus any difference in taxes and other fees the airline will decide to so (also expect to have this take an hour or two, so best to be quite early for such flights). Sometimes the agents miss it, but 99.9% of the time it will be caught these days (the agent will be in big trouble, the back office will catch it later).