Is it possible to bank overtime?

Yes, it's possible.  It takes a couple of extra steps, but works very well for me.

As you know, the Bank Hourly payroll item is native to QB and is what is used to accrue hours for each employee.  But to accrue hrs way QB would like you to, which is to manually enter the hours in the employee's payroll record in Payroll Info - Accrual Hours is cumbersome.  I use the following workaround.

The Bank Hourly Item, which when used, only records Bank time that is paid out in any given pay period, and reduces any accrual hours you may have, which are positive.

1. Create a payroll item called something like 'Bank Hrs Accrued', and link it to the same wage expense account as your regular hourly item.
2. Create another payroll item called something like 'OT Bank Differential', 
3. When entering overtime into the timecard, enter the regular hours as you normally would
4. Tab down to the next line, and use the PR item 'Bank Hrs Accrued', for the exact number of OT hours the employee worked.
5. Tab down to the line below that, and use the PR item 'OT Bank Differential', and enter the OT multiplier of the OT hours worked.  For example, if the employee worked 2 hrs of overtime, enter 1 hr in the differential item (based on 1.5 OT rate).  If the rate is different, use that rate.  For example, double time, you would enter 2 hrs of OT and then 2 hrs of OT differential, etc.
6. At the time of creating pay cheques, whatever number of hours are in 'Bank Hrs Accrued' and 'OT Bank Differential', you will make a negative entry to the QB native PR item 'Bank Hourly Rate' (you may have named it differently)
7. Note that while in 'Review Pay Cheque', as soon as you enter the negative 'Bank Hourly Rate' item, the Bank Avail (HR) at the top right of the pay cheque, now shows a positive number of Bank hours available.
8. Conversely, when employees take time off and indicate they want to use their banked time, simply enter the 'Bank Hourly Rate'  PR item into their timecard for that day.  When you review their next pay cheque, the Bank Available (Hrs) will be reduced by the time taken as Bank Time.

When you are pulling reports and only want to use the OT hours, you can filter the report to exclude the 'OT Bank Differential' item and only use your regular OT Hourly item and the Bank Hrs Accrued item, to get the actual number of OT hours worked.

*Note that when accruing bank time this way, the area on the Paystub which has an 'Earned' column and 'YTD Used' column, will be invalid info.  Only the 'Available' column will be correct.  If you choose the more cumbersome method, which is to keep track of all of the OT hours that someone wants to bank and manually enter them each period into the Accrual Hrs area of the employee's PR profile, then those columns would be correct.  But if you have many people to pay and track bank time for, the method above works very well and is much quicker because the pay cheque is populating entirely from the timecard, except for the one line entry you will make on the pay cheque to 'accrue' the banked OT hours.