Shapefile to Geodatabase Questions

I have a bunch of data in shapefiles, that I would like to import into a geodatabase.

When I do so, the blank fields do not turn to NULL in the geodatabase. How can I fix that?

I would also like to change all of the "0" fields to NULL. Is there a quicker way to do that than go through each one individually?

Once my shapefile is in a geodatabase, can I add domains and subtypes? Or does that have to happen before data is entered into the geodatabase?

I've got several books, but they all start with creating a geodatabase from scratch, rather than fixing up a geodatabase that has been populated with existing data from a shapefile...

Thanks for any help!