Custom Action to add more users to approve the workflow task

In Nintex workflow environment inside SP2013, when delegate a workflow task using Flexi Assign to another user, the first user will lose the ability to approve that task after he/she become available again.

So I'm trying to create a custom action to simply add more users to the current workflow task then any of (first user or new users) can approve this task.

I'm using VS2015 with Nintex workflow SDK and SP2013 . Any help will be appreciated.

Thanks