Can I add a billable expense from a prior year to a current year invoice?

yes you can add them

billable expense charged to the customer is income not expense, in company settings you should set it to record billable expense as income and choose the income account

Thank you for taking the time to reach out to us here on the Community page, @KrisG9988.

If the expenses have billable selected to a customer, then yes, these can still be added to the invoices dated this year. If they don't, you can edit those transactions and put a check-mark on the Billable column and select a customer. Just be sure not to edit the amounts or dates as this will affect the reconciliation.

To modify the expenses:

1. Click Expenses at the left pane, then Expenses so you'll be routed to the Expense Transactions.
2. Filter the date, then look for the transaction.
3. Click on the drop-down arrow under Action and select View/Edit.
4. If the expense was not marked as billable, check the box under the Billable column, then select a customer under Customer/Project.
5. Save the transaction and repeat the steps for the rest of the billable transactions.

Once completed, you can now add the billable transaction to the current invoice. For your guide, check out this article: Enter billable expenses.

You can reach out to me if you have any other questions. I'm a few clicks away to help. Have a good day!