Yes. Once you have an active subscription for a customer, you can attach one-time charges to their account.
You can work around the requirement for having to have a subscription by setting up a Subscription for a $0 recurring charge, although you will want to have email receipts turned off for your Site if you go this route - periodic emails showing your $0 renewal can be confusing to Customers.
Another way to achieve a one-time purchase is by setting up a specially configured "One-Time-Purchase Product"
We plan to offer formal support for One-Time purchases in the future.