Using the extension CiviVolunteer, assigning a volunteer to a company, not an event
Earlier this year, the CiviCRM community crowdfunded $30,000 to build CiviVolunteer 2.0, which includes the functionality you're describing.  See here: https://civicrm.org/blogs/ginkgofjg/civivolunteer-20-whats-in-the-works.  The beta version should be out in 4-5 weeks.

For now, the workaround is to create a bogus event to attach volunteers to.