It is possible to add custom fields to the workgroups (Desktop>Settings>System settings>Custom Fields, enter SONET_GROUP as the Entity field), but the support of the 'Bind to CRM elements' field type is still in the planning stage.
This form is built with the bitrix:tasks.task.edit component. So you should copy its template to the site template and edit the template.php file (of the copy). The "CRM Items" section is displayed with the following code:
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