Your Ubuntu boot loader should be able to add Fedora to the menu. After Fedora is installed boot Ubuntu and run sudo update-grub (or whatever the Ubuntu command is these days). Prior to that, during the Fedora install, choose to install Fedora's boot loader to the root partition, the same one you're installing Fedora to, instead of the MBR. That way you're existing Ubuntu boot loader won't be overwritten.
By the way, welcome to Fedora Forum