When you create a work group you have to specify a default work queue and when you create a work queue you have to associate it to a work group. This doesn't mean that you cannot associate multiple work queue to the same work queue. It only means that a work group have a default work queue to route the work when there is no specific work queue to assign the work to. The default work queue is sort of a miscellaneous work queue where user can pick different type of tasks from.
To associate multiple work queues to the same work group you just need to edit the work queue work group field or set it when you create the work queues to the work group you need to.
Ideally you shouldn't customize the OOTB rules. One way as to save as the section of Workgroup form to your own rule set.. Give a grid like structure to add multiple workqueues (Which you have to create first). Once you save the section rule , you need to save all Work queues and workgroup on click of save button. It would be a tricky one and quite complex to achieve.
Sorry, I'm not sure if I understand. A work basket is another name for a work queue, the object rule is the same.
Despite in the work group form you can only indicate one default work basket/queue. The relationship between work group and work basket, it is one to many. A work group can be associated to many work basket/queue but a work basket/queue can only be associated to one work group.
If you need to display a form with the relationship, all you need to do is create a data page sourced by two report one to get all work group associated to ruleset within your application and the other getting the work basket/queue for each work group. Then present the data in a data grid or your preferred UI element sourced by the data page.
My path was almost same but if we are modifying the OOTB Workgroup form then we should have only have a multi-select control sourced with a Data Page which brings all WB's related to your ruleset. The multi-select control gives a more better UI Experiences to Users to select multiple WB's rather than grid. Then when we save the particular work group rule , we can call custom activity to save all WB's selected to the respective WB.