Possibilty to flash a TWRP/CWM on a smartphone that doesnt have custom recovery? I have a Huawei Y7 Prime that doesn't have any released custom recoveries. Is it possible to flash or even just boot TWRP/CWM for another smartphone (eg. for Huawei P8) on my Y7 Prime? Is it possible to do this in general? and if no, is there any way to root my device without custom recovery just by using fastboot or...?!
Simple answer NO

It is not possible - recovery is device specific and unless it has been developed for your device, you are out of luck

You can try one-click apps that root, if it works for you (like Kingo root etc but be aware that they usually have their own set of problems)