How to allow portion of expense payment to be marked as ‘personal’ (non-business)?

You could enable enable Personal field at the expense type level. 

The steps are:

In Admin, under Expense-> Expense Type, select an expense type an in the personal field there are three options:

• No, does not allow any portion of payment to be marked as personal.
• Yes always on, always allow portion of payment to be marked as personal, for Out of Pocket expenses and credit card payments.
• Yes only for credit card payment type, only allow portion of payment to be marked as personal for credit card payments.