It is actually possible! If you have a site running on InCMS (e.g. mysite.com), you can setup a subdomain with your hosting company and install WordPress on this subdomain (e.g. blog.mysite.com). This has to be done outside of InCMS. It only works if you add the domain with external nameservers in InCMS (in our example: mysite.com needs to be added with external name servers).