Yes, you can create time/expense entries on behalf of another firm user.
This comes in handy for situations such as a paralegal entering time for an attorney. Even though the paralegal is entering the entry into MyCase, it should still be tied to the attorney, not the paralegal.
When creating a time/expense entry, there is a field called User (pictured below). You will see a dropdown menu that has the names of all the firm employees on your MyCase account. By default, your name will appear here. Simply select the name of the person you'd like to add the entry for.