Not sure you could just jump from one plan to another mid flight?
This probably depends on the type of GPS but it's certainly possible to change flightplans in flight using 430/530.
During pre-flight planning I use Google Earth to obtain my lat/long co-ordinates, scrolling the pointer to the desired location and reading off the bottom of the page. They are always within a couple of metres or so.
I use my own waypoints for routing through and around controlled airspace and landing sites; they are used bioth IFR and VFR.