I've used platforms similar to this one:
But after looking around a bit, I saw this product, which looks a little more rugged since you screw your actual cleats into the platform to clip it in rather than relaying on the molded plastic of the platform:
But have found that these platforms are really a temporary solution meant to let you ride to the corner store, not meant for long term use. Since the platforms clip in like a shoe, they can clip out like a shoe and they float around a bit, so don't feel super stable. Also, they make the pedal top heavy, so the platform always ends up on the bottom and you have to flip it around (gently) so you can get your foot on it - if you hit it too hard when you're trying to flip it, it's easy to knock the platform off.
Your best bet is to try to get the stuck pedal off and put on the pedals you want to use. Here are some tips on removing a stuck pedal: Removing a stuck pedal
I've never had to resort to using heat, but I've been told that it's a sure fire way to loosen a stuck pedal because aluminum expands more than steel when heated.
But first, as mentioned above, make sure you're unscrewing in the correct direction, and use a good quality pedal wrench.