In looking at ways to efficiently load in spreadsheets for leads management and ways to mass assign, and add needed information, it seams like the best approach is to do it through contacts. Currently that is the only place that has a bulk editing feature, and it allows for many more fields than does the leads inbox. 

 

Example: 

https://www.loom.com/share/a132ba0a744642c181615d43366eca36