Link expense to existing invoice?

Link expense to existing invoice?

Is it possible to simply link an expense to an existing invoice?

Many times I have small expenses that I would like to hold and place on an existing larger invoice. Is this possible? If not, then a check box in the expense detail stating "previously invoiced" or a text field where we can place the invoice number manually would be nice. This way, I wouldn't have so many un-billed expenses and I wouldn't have to send multiple small invoices out to my customers.