Attaching job to the existing item on customoer's transaction

When we create a new job(project) from new sales order, everything works fine with the following process:
1. Select customer
2. Create sales order
3. Select item with "Create New project" and some tasks filled in
4. Approve sales order
5. Go to "Create jobs from sales order"
6. Create new project

In the above case, project is assigned to the specific item on the transaction or sales order.

However, we could not figure out how to assign already existing job to a particular item on already existing transaction. Is there a way to do it using UI and/or custom script? There is a drop down list that allows to assign project (job) to the whole sales order or invoice, but not to a particular item on the invoce/so. Am I missing something? How do they linked? Is there a special field in the job object? Is there a sublist to which we would be able to add an record and specify transaction ID it is linked to?