In order to migrate the Web subsite, please promote it to the target Sharepoint site this way: select the Web site, then click on Map Web on the right pane. In the following dialog, please check "Promote this web to a site collection".
The trick here that after doing so, ODMSP is creating a virtual source Sharepoint Site in the list with Migration Type column showing "Promote". This is the site that should be migrated after.
