Oh, yes, sure. Get root, find /system/etc/xml/multiwindow_whitelist_apps.xml file, open it with any text editor that supports root and put there something like
Code:
<mw_nonsystem_app package_name="xxx.yyy.zzz" />
where
xxx.yyy.zzz is you app package name. You can see you package names with 3C Toolbox or any other application manager, or get them from /data/app by taking the application folder name and removing the last 2 symbols from there (was: "com.mypackage.myapp-1", will be "com.mypackage.myapp") or see them on Google Play website in the URL of your application.
After you edited the xml, reboot and your app will support multiwindow mode. If it still doesn't, make sure you didn't make a mistake in the package name (Pro versions of free apps can have different package names) and go to Settings->Apps->Menu->Show system processes->HwDualWindowLauncher, clean its data, reboot and try again.