I am having the same problem. Where did you find the “add to invoice” in the expense edit screen? I am not seeing it in the “more actions” box. I too want to be able to add tasks and invoices to the same invoice. I am using the self-hosted version of Ninja. I need to update to the current version as soon as I get my IP unblocked from forgetting my cpanel login. (oops) Is this ability in the current version?
Hmmm. . . I created some tasks for this client, created some expenses and applied them to that client. I went to the tasks and created an invoice, then I went back to the expenses to see if I could add them to the newly-created invoice, but I did not see the option to do that. I’ll double-check once my host lets me back into my site. I’m wondering if my order of operations was wrong, and you can’t add expenses to open invoices if they are created AFTER the expense was created?