While it is not possible to have a single line item that encompasses all the employees and vendors, it is possible to have one line item for all employees and another line item for all vendors.
In BillQuick > Allocation & Forecasting screen, with the “View By” set to Project, you can use the 'All' employee group or the 'All' vendor group when setting up a task allocation, as shown here:
Additionally, you can combine this allocation with a project rule (Projects > Details screen) that locks time entry to only the allocated hours for a particular activity item. This greatly simplifies how many hours you are allowed to enter.