From the FAQ's:
Q: The Wave/Field I am qualified for is full. Can I sign up for a Wave/Field I am not qualified for?
A: No! As obvious as that sounds enough people have asked about this that it is now in the FAQ. And this goes both ways: slower runners cannot sign up for faster Waves/Fields and faster runners cannot sign up for slower Waves/Fields. See the first question in the FAQ about what happens if you do this! As for the “why” we are trying to spread the runners out over the single track trail as much as possible so everyone can have an unimpeded race. That would not happen if we allowed the overflow from one Wave/Field to spill into the other.