How do I enter retroactive or year end taxable benefits so that the tax is calculated on the benefit?

How do you enter a random or retroactive taxable benefit paid to an employee. I pay for some random taxable benefits for my employees, including prescriptions, sometimes a tank of gas, eye glasses etc. I keep the costs of the items in their own GL and total them at the end of the year. Easy enough to edit the T4 to include the amount for each employee, but there is no tax, CPP, or UI deductions. I always end up with a CRA PIERS audit saying that I owe tax, CPP and UI on the added amounts. I want to 'pay' into the employee CRA account, the tax, CPP and UI so that their CRA requirements and mine are met. I have tried adding the taxable amount in the benefits tab and only enter the benefit amount once on the last paycheque but that results in a much higher tax calculation than is required and the employee's last paycheque is reduced substantially. I tried doing a benefits only cheque, which ends up as a negative paycheque. This actually reduces the total wages. eg start with 50,000 total pay. Do a supplementary 1000 benefits entry and manually enter the tax ,CPP, UI as calculated on the CRA web site. Lets say that the total deductions are $250. This then results in a negative pay cheque of 250. The result is that the total earnings actually drop by $250 (49,750) when they should actually be 51,000 or at least not change and remain at 50,000 with extra deductions taken, and I can then amend the T4. Does anyone have a solution to enter the taxable benefit without reducing the employee wages so that payroll calculates the tax, CPP, and UI and the employee (and my) CRA accounts are credited properly to cover the taxable benefits deduction requirements.