I recently built and tiled a walk-in shower . I have now found that the drainage angle is not enough and the water lays on the floor instead of draining properly. Can I use a schluter floor tray and tile over the existing floor without having to tear out the entire base?

No, not really. The schluter tray is designed to be used as part of a complete system of topical waterproofing and gets used with their drain as well. It's pretty much useless without the Kerdi membrane covering it, though I suppose it could be used as an expensive presloped base for a conventional mud pan with liner.

All hope is not lost, though. Please detail your shower pan and wall construction method and materials and maybe we can come up with a workable solution.

I used water sealing material, just not the shluter system. I don't want to remove the entire base as it was very expensive tile and I used apoxy grout to seal. I am curious as to why I can't use thinset to relevel the floor and then use this tray and tile over the tray following their directions and using a piece or their sealing mat. Could this work?

I have seen it done many times before as old shower floors were tiled over with new layer of tile. It was done for the looks or to correct slope problem, like yours. It always worked.

You are basically adding second layer of tile over existing layer. No water proofing is necessary. Install new 4 1/4" square drain frame with new round drain inlet cover. Remove strainer from old drain and install new one directly over it. Use fortified thin set. You need very little to create new slope. I am sure it won't be more than 3/8".