Add a Field to the Lead Conversion Form

1. Navigate to Customization > Lists, Records, & Fields > Entity Fields > New.
2. Enter the name of the field.
3. Choose the type of the field.
4. Under the Applies To tab, tick the box for Customer.
5. In the Display tab > Subtab field, choose Main.
6. Hit Save & Apply to Forms.

Note:  Only the fields in the Main section of the Lead Form are automatically added in the Lead Conversion Form.