The manual (Sage 50 User Guide) doesn't describe this in detail, but it turns out that one can put in a vendor bill for a non-stock item (or several non-stock items) by charging an expense to the relevant GL account, bypassing the first three columns of the Purchases line-item entry dialog. Perhaps this could be documented on or about page 124 of the manual.
Types of expenses for which this might be useful are electricity, internet, rent and so on.
Where the expense is billed before it is paid, accrual accounting practice suggests it should be entered as from when the invoice came up or when the goods or services are received, not from when the bill was paid.