Are you looking to do this to
1) allow users to have a "bucket" to submit non-project time to on their timesheet or
2) are you just needing to block out some time so that project managers don't assign them 100% to project work.
1) In order for a user in Clarizen to submit time to a "bucket" in their timesheet, that work needs to be created in the system in the form of a project, milestone or task, and assigned to them. So in this instance you do need a Admin project or task with all users assigned
2) In this case you could set the default members availability (%) for a new project to less than 100% meaning that PMs can only allocate resources to projects at that % before they are marked as overloaded. E.g. Set default to 80% (meaning that 20% will be spent on none project work) and when a resource is allocated work exceeding that 80% they will be marked as overloaded