It should be no different than creating content type from any other built in content types. It is little tricky because the Workflow Task Content type is hidden.
Following is the Workflow Task content type from which you need to derive your own content type:
<ContentType ID="0x01080100C9C9515DE4E24001905074F980F93160"
Name="$Resources:WorkflowTaskIP_Name;"
Description="$Resources:WorkflowTaskIP_Description;"
Group="_Hidden"
Hidden="TRUE"
Version="0">
<FieldRefs>
</FieldRefs>
<XmlDocuments>
<XmlDocument NamespaceURI="http://schemas.microsoft.com/sharepoint/v3/contenttype/forms/url">
<FormUrls xmlns="http://schemas.microsoft.com/sharepoint/v3/contenttype/forms/url">
<Display>_layouts/WrkTaskIP.aspx</Display>
<Edit>_layouts/WrkTaskIP.aspx</Edit>
</FormUrls>
</XmlDocument>
</XmlDocuments>
</ContentType>
And following is an example of Publishing Approval Workflow Task that derives from above content type and ships with SharePoint:
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
<!-- ReviewPublishing -->
<ContentType ID="0x01080100C9C9515DE4E24001905074F980F9316000A245BAB39C6543159300E33084BA0409"
Name="Publishing Approval Workflow Task (en-US)"
Description="A work item created by an workflow that you or your team needs to complete."
Group="_Hidden"
Hidden="TRUE"
Version="0">
<FieldRefs />
<XmlDocuments>
<XmlDocument NamespaceURI="http://schemas.microsoft.com/sharepoint/v3/contenttype/forms/url">
<FormUrls xmlns="http://schemas.microsoft.com/sharepoint/v3/contenttype/forms/url">
<Display>_layouts/WrkTaskIP.aspx</Display>
<Edit>_layouts/WrkTaskIP.aspx</Edit>
</FormUrls>
</XmlDocument>
<XmlDocument NamespaceURI="http://schemas.microsoft.com/sharepoint/v4/workflow/forms">
<WorkflowForm xmlns="http://schemas.microsoft.com/sharepoint/v4/workflow/forms">~sitecollection/_catalogs/wfpub/Publishing Approval/ReviewPublishing_Task_1033.xsn</WorkflowForm>
</XmlDocument>
</XmlDocuments>
</ContentType>
</Elements>