welding vertices? No I don’t believe so. ZBrush is very dependent on the order of the vertices and given that welding often messes that up, no, I don’t think you can do that. In fact if you get half way through a sculpt and discover that you left a hole somewhere or something, you may have to do some retopologising after you repair the mesh in your modeling app.
Recenter the pivot? Well, I think that’s possible using ‘Transform > Set Pivot Point’, but what I normally do is export the lowest sub-d level to my modeling app, place the pivot where I need it, then bring it back in to ZBrush. I do the same thing if vertices need to be welded, though I try to handle all of that before I ever get to a sculpting session.