As of now (6.1+) To include external schema into a local schema you have two options:

1. Use new XML category -> External XML Schema Document(s) to add a local instance

2. Manually edit the source of the schema to import an external schema by adding the appropriate code to the source, ie.
add <xsd:import schemaLocation="http://schemas.xmlsoap.org/wsdl" namespace="http://schemas.xmlsoap.org/wsdl"/> or
something to the source code.

It would be very nice to have the import schema dialog be able to retrieve external schema avoiding the necessity of the
other options.