Allow Updating Existing Users while bulk import

Problem/Motivation

I have a need to run a bulk import where some users already exists and others don't. For the ones that exist, I'd like to update their information with what's in the spreadsheet / data from DB.

Proposed resolution

I've posted a patch to get the ball rolling, and to see if this feature is OK to flesh out a little more. Below are the remaining tasks that could make this update more valuable.

Remaining tasks