Yes, define a custom field that is linked to your Username, and set the custom field name to email.
To set up the required custom field, edit your vault item, scroll down to the “CUSTOM FIELDS” area, click the dropdown menu below where it says ⊕ New custom field to change the selected value from “Text” to “Linked”, and then click the ⊕ icon to create a linked custom field. You should see a new custom field with a red ⊝ icon, a Name field, and a value that says “Password”. Click where it says Name, and when you see the blinking cursor at the start of the Name field, type email. Then click the dropdown menu below the name field, to change the selected value from “Password” to “Username”. Finally, click the “Save” button in the upper right corner of the browser extension.