Creating a custom field on expenses and bills isn't possible for now. I know how much custom fields can help to add more details to your bill and expense form.
In the meantime, I suggest submitting feedback to our product developers. In this manner, they'll know that this is crucial to your expense and bill forms and might add this in future updates.
Here's how:
Select the Gear icon at the top, then Feedback.
Enter your comments or product suggestions. Then select Next to submit feedback.
For more tips on tracking your expense transactions, I recommend reading these articles: