I believe you'd have to modify the XSD manually according to your needs. Using the "choice" tag would allow the modeler to create import and export mappings with inheritance.
For example, if you consider the case where you have a general object Car and two specializations: SUV and Sedan
When you export the XSD that exposes a car, which will have a line like:
<xsd:element name="Car" minOccurs="0">...</xsd:element>
Inside these element tags you'll see each of the entity's attributes. Around the element tag, add the choice tag:
<xsd: choice>
<xsd:element name="Car" minOccurs="0">...</xsd:element>
</xsd:choice>
Then you can add the specializations of Car. You could auto-generate the XSD schema for each of these elements using the modeler, or create them manually:
<xsd: choice>
<xsd:element name="Car" minOccurs="0">...</xsd:element>
<xsd:element name="SUV" minOccurs="0">...</xsd:element>
<xsd:element name="Sedan" minOccurs="0">...</xsd:element>
</xsd:choice>
When you importing this XSD into the modeler and create an import or export mapping, the choice elements can be mapped using inheritance (note I excluded SUV here):