Please add support for XML Catalogs (http://www.oasis-open.org/committees/entity/spec-2001-08-06.html). A catalog can be used in two different, independent ways: (1) it can be used to locate the replacement text for an external entity, or (2) it can be used to locate an alternate URI reference for a resource.
When you create a new SOAPUI project based on an existing wsdl file, the location of the xsd imports can be located by looking at the definitions specified in the catalog file.
By the way, all JAX-WS impementations support this feature, so it might be not too hard to implement it if you can use one of these implementations.
For a use case of this, look at: