The invoice editor has an Add unbilled expenses section. It lists your reimbursable expenses that have a Contact, are in the invoice currency and are not on an expense report or another invoice. Choose the invoice Contact, select expenses and add them as draft line items.
Each line keeps a link to its expense. The expense shows the invoice it is on, and it cannot be changed or deleted while it is on an invoice. Issuing the invoice marks the expense as billed. Removing the line from the draft, or deleting the draft, releases the expense. An expense on an invoice cannot be added to an expense report.