The short answer is "No, unfortunately not."
Navistream accounts belong to a company, or tenant, and are self-contained.
While your username needs to be unique to others on the system, you can only log into a tablet that belongs to your company.
If you're looking to drive for someone else who uses Navistream temporarily - short or long term - they'll need to give you one of their tablets and create an account for you on their end.
Make sure you make a copy of the last two weeks of logbooks from your own account before using another company's tablet as their account won't have your information! Print these and keep them on hand.