It is currently not possible to extend a SAPUI5 (Fiori) App with Mendix. You can integrate into it via services if you require.
By creating a new Mendix application via the "Create App" Flow it is possible to choose an application and use Fiori Design patterns in your Mendix application. It is also possible to have Mendix as a Fiori tile in your launchpad and use the SAP single Sign on service to automatically log your users in. There are some sample applications on there to help you understand how to build a Fiori Designed application.
Mendix also has pre-built connectors to allow you to configure your application to utilize many of the SAP products.
These applications can then be hosted if you require on the SAP Cloud Platform. You can set this up in your project settings or choose an SAP app during the create app process.
I hope this helps?