It is indeed trivial, and I've done it once or twice to fill in sections where I was riding but my GPS batteries had died, thus avoiding a big ugly straight line mid ride.
But what may be non-trivial would be to fake a ride going at a particular speed, with realistic accelerations and decelerations and stops. You'd need to take into account things like the sampling rate of your GPS and I imagine (but I don't know) that a faked ride could be distinguished from a real ride by various tell tale signs so I expect it would get a bit tricky and really, life's too short to care about such things. I guess that's why sites like Digital Epo came into being, because it's actually not trivial if you want to do it well.