Hi Sandy,
This is definately possible with some customization. To start, you would need to determine on which action the Purchase Order would be created. You could do this on the save action, and then then either set a new Yes/No field if the Purchase Order should only be created once, or you could set a new dropdown field linking to the Purchase Order which you could use to update the it whenever the Sales Order is saved.
Since the Purchase Order and Sales Order tables do not have identical fields, you would need to determine which fields would copy over. When you are creating the Purchase Order you could include logic to set the preferred vendor. Lastly, you would have to copy over all of the line items.
If you have any questions about this just let me know. If you would like have one of our consultants (such as myself) help you on this please reach out to your account manager. For more information on consulting please see this page http://www.method.me/support/assisted-services/.
Regards,
Russell