First, you need to remove the bio-balls. I would suggest doing this slowly, and only removing 1/3 per week to allow the rock and sand in your tank to pick up the slack that your bio-balls were handling.
Next, you would need some modifications of that sump to allow you to use it as a fuge. For it to be a fuge, you will need it to hold water, so it looks like 2 options(short of setting up a seperate tank as a fuge).
1 - seal the current hole in the bottom of that baffle so that section will fill with water. You need to make sure that water can flow over the baffle without spilling out of the sump. If the baffle extends to or over the top of the sump, you will either need to drill lots of small holes, a couple of large holes, or cut the top off to allow water to flow over, or through, without spilling out of the sump.
2 - Build, or have built, a small tank that will fit into that section of the sump. The same issues apply about water spilling out of the sump. This option is nice because you could build the tank taller to hold more, and just drill a hole near the top, use a bulkhead and some PVC pipe, and have it drain into the other section of the sump without messy modifications. I would suggest using some additional supports under this mini tank. HTH.
