If you don't need the features of Helpdesk Teams, just create tickets without assigning them to a team and they will be visible by all users who have access to the Helpdesk App in every Company.
If you do need the features of Helpdesk Teams, create ONE, attach it to ANY Company, and modify the relevant record rules to share tickets between companies:
This record rule says "only show tickets if there is NO COMPANY (no team) or if the Company is in the list of Companies the user currently has checked on.
Change it to this:
Also, change these in the same way:
Helpdesk Teams give you a TON of value, so I would recommend the second option.