How can I add holiday hours to all employees?

Are you looking to add holiday hours to all employees at one time? If so, there's no option to do that at the moment. You'll need to add the holiday pay type one by one per employee.
 

Go to Payroll > Employees > Select an Employee > Job & Pay section > scroll down to Additional pay types > select Holiday Pay > Save. Repeat this for each employee.
 

Once that's done, when you run payroll, the Holiday Pay line will appear on each employee's paycheck.  Enter the holiday hours there alongside their regular hours. 
 

As for adding a holiday balance, could you clarify what you're looking for here? Are you wanting employees to have a bank of holiday hours they can draw from throughout the year, similar to how sick and vacation time works? That'll help me point you in the right direction.

Let me know your thoughts. 

Yes, but after the holiday line appears, is there a way to enter holiday pay for all employees without having to enter it individually?  Is there a way to set up a company holiday schedule so it automatically calculates for all active employees without us having to remember to put in the holiday pay?

There is no single button that automatically pushes holiday hours onto every employee's timesheet or paycheck at once, but there are two practical approaches depending on whether you use QuickBooks Online Payroll or QuickBooks Desktop Payroll. The fastest route in either version is to enter the hours during the normal pay run rather than trying to pre-populate them individually.

QuickBooks Online Payroll: When you create the paycheck for the holiday pay period, click into each employee's timesheet or paycheck detail and add a new line. Select your Holiday Pay pay type (or create one under Payroll Settings > Pay Policies > Holiday Pay if it does not yet exist), enter the number of holiday hours, and save. If you use QuickBooks Time, you can have employees submit their holiday hours through timesheets and approve them in bulk before running payroll.

QuickBooks Desktop Payroll: Open the Enter Payroll Information window for the relevant pay period. For each employee, double-click the Holiday (or Vacation/Sick) column and type the hours. If you have many employees, you can use the Timesheet feature instead: go to Employees > Enter Time > Use Weekly Timesheet, select the employee, choose the Holiday payroll item, enter the hours, and repeat. Desktop also supports importing timesheets via CSV or Excel using a third-party tool if you are adding hours for dozens of employees at once.

If you want holiday hours to accrue automatically going forward, set up a Holiday Pay policy in your payroll settings so the pay type is available on every paycheck without manual setup each time. This keeps the pay item consistent across all employees and simplifies reporting at year-end.