I don’t know if it’s going to be possible to salvage this model. It depends upon the nature of the symmetry problem.
If the problem is due to a positional variance, then you should be able to resolve it by pressing Tool>Deformation>Unify.
If the problem is due to a rotational variance, it’s going to be almost impossible to resolve. You might be able to pull it off by using the Rotate deformation, but you’d have to hit the exact angle. Not very easy to do!
Alternatively, you could try fixing the problem in your original app. If you can track down the cause of the problem, then you can export an accurate version. In ZBrush you would go to level 1 of your model and import the new version over top of the old one. That should resolve the problem.