Hey Breno,
Gregor Betz explained that in another blog post (see http://www.argunet.org/2013/06/12/how-to-reconstruct-linked-convergent-and-serial-arguments-with-argunet/). What you want to do is called 'linked argumentation' in the blog-post.
Best regards,
Sebastian
Hey Breno,
you got it! What regards your remaining question: Whether you define a new equivalence or add the conclusion/thesis as premise, depends on whether you already added an argument-reconstruction for the argument (b) with the corresponding sentences as premise or not. (For details on that you can refer to the Argunet-Help - e.g. http://www.argunet.org/working-with-argunet/ch03s02.html)
Best regards,
Sebastian