Holacracy is for people working in an Organization for its purpose on a day to day basis. A volunteer working only 1 or 2 days per week is not a real partner and shouldn't be assigned to a role - meetings takes a lot of time already.
A contractor works for another company and is not contributing to the same purpose. We had the experience of both and giving them a role (full authority), including them in governance and tactical meetings doesn't make sense and doesn't work. You can't give full authority to someone external.
The right move would be to assign a partner to a role and then leaving him/her the choice to ask a volunteer or to call a contractor for some projects. He then process tensions directly with the volunteer/contractor.
For instance, I have a graphic design role but I am not especially good in design, I just hire a sub-contractor, deal with it normally.