How to add a custom expense field to Zoho Budget - Expenses?

How to add a custom expense field to Zoho Budget - Expenses?

Out of Box, Zoho Books' budget expense categories are limited. I like to add a new expense category into budget/Expense items like 'Travel - Air',  ' Travel-Taxi'. Appreciate if anyone can provide steps to add a new custom filed into Books --> Budget --> Expense. Thank you