Basically yes they are not compatible. FPGUI has a different widgetset model.
The long answer is probably that, provided you give up on the advantages of FPGUI, it should be possible to hack it into your application, but that would also mean you have to include lot of code, if not all, from the widgetset your version of Kmemo was built with. So a complete GTK2 dependency, lcl dependency, lcl components dependency....Get it?.
Your application then becomes huge and very hard to maintain.
So yes, it is not compatible and no, with sufficient skills you can hack away to your liking.
Just don't do that. Use a standard Lazarus widgetset if you need kmemo.