unfortunatly, i don't think its possible on that level, you have to fly for a longer length than yoshi can float.
I've opened the cape exits with Yoshi. I'll asume you're referring to the exit for Soda Lake, in which case, it is very possible to do. You have to bounce off the enemies/objects to keep Yoshi flying through.
What he said ^^^
You can also bounce off them as Mario/Luigi, with the A button spinnign jump. Fun.