Yes.
Simply go to Deals and find the account that has the closed deal that you
would like to attach the project to and open it. Once it opens to the right,
click on the drop down arrow for Deal Name and open the closed deal. Once
that is opened, click on the Projects tab towards the lower half
of the screen. Click the “+” sign beside Projects and add the project and
click Save. The project will then be attached to the closed deal.