Yes, you can use and in-line callout that references a JSP included in your web project (so that the virt server can execute it) that itself returns back a portion of your XMLSchema to be used to generate our web-based form.
The JSP can callout to your external DB, fetch whatever values it needs, and then once again return them as fixed values in your schema. Take a look into this. This is the precise use case intended for in-line callouts (please also note that in the future we intend to support calling out to Alfresco web scripts to accomplish the very same thing).
Kevin