Thanks Aika.

I actually worked it out. 

1. Go to Employees tab.

2. Select Payroll Settings and then Chart of Accounts.

3. Click on Import Accounts at top right of screen.

4. "Turn on" the account that you want to add to the payroll chart of accounts.

5. Save accounts list.

The account will now show under the recommended accounts to choose from when you are allocating an expense account in the payroll deduction categories.