Tag and Issue to a Task which has Already been Created

We have a request queue which receives requests for new pieces of software development for our platform from across several teams. We can convert an issue from this queue into a task, but cannot tag an issue to a task which has already been independently created in the development workflow. In this scenario, we wouldn't want to create a new task because it already exists, but having the ability to link that issue to the existing task would be of great benefit. Thaks!