How can I prevent echo from footsteps on a hollow, temporary stage?
Are you sure echo is the main problem?  It shouldn't echo much unless it's on a hard floor.  Setting it up on top of something soft like carpet remnants should pretty much eliminate echo under the stage.  But even if there is some echo under the stage, it's unlikely to be the loudest sound you're hearing.

Maybe you're hearing the clattering noise that happens when the stage's joints are compressed and different parts clap together.  This type of sound is going to be a lot louder than echos under the stage.  You may need to identify which joints are to blame and stuff bits of felt in them.

One more suggestion is to check your dancers' shoes.  Hard soles landing on a hard stage are going to make noise.  Padding the stage and/or the shoes could help.  Just make sure they'll still have the traction they need to dance safely.