Connecting an Issue with an already set up Project

Description - PLEASE add functionality for us to connect an issue with a project that has already been set up. 

Why is this feature important to you - We Forecast a good portion of our work so that we have placeholders during our heavy seasons. When we receive issues for these requests from a queue, it won't allow us to connect the two while bringing over the custom form and attachments. 

How would you like the feature to work - When converting the issue to a Project, an option should be "convert to established project" and we should be able to add to the project we already have set up but also bring over the custom form and attachments from the Issue. 

Current Behaviour - this doesn't exist. As a workaround, I'm having to convert the issue to a new project, clearing out the timeline that populates, copy Forecasted timeline over to the new project then I have to delete the project I had in Forecasted. Very cumbersome workaround.