The best way to add an expense to an active job is on the job's completion page. From the active job page, click 'Complete' button which will take you to the completion page. In the Expenses Detail section, click the '+ Expense' button. After you've added the expense(s), click 'Complete' to complete the job or click 'Update Visit' to save your updates but keep the job active.
Alternatively, you can also add expenses to a job by creating a new purchase order and associating it with either the job or the visit. This is particularly useful if you find you need to add an expense to a job that has already been completed or approved. Keep in mind that if you do not want to affect the job costing inside SingleOps, you should associate the purchase order to the job rather than to the visit. The expense will sync to QuickBooks as a bill in either case; only SingleOps makes the distinction within its job costing.