You wouldn't be able to. To use the CC catalogue, you would need to buy Lightroom 6.
When your subscription ends, you will be able to continue using your CC catalogue. You just won't be able to go into Develop or Map. That may be enough for you.
But if you go back to 5.7, probably the best route is to select all your photos in Lr CC and do Cmd S / Ctrl S to save your work back to the images. Then import everything into 5.7. You'll lose stuff that isn't saved to metadata (stacks, flags, collections, history steps).