Add Projected End date to mass update

Summary:

Under Create Mass Assignment update we want add new fields like Projected End Date

Content (required):

Under Create Mass Assignment Change -> Assignments Details business wants include more fields like Projected End Date, Descriptive flexfields and others,

How to achieve it?

Version (include the version you are using, if applicable):

23b

Code Snippet (add any code snippets that support your topic, if applicable):