The HiSuite doesn't have a Music/MP3 tab. I've checked the interface and this option isn't there, however, since you have a Huawei P9 Lite, simply connect it to PC and enable USB Connection when prompted, then you can add your music to the Internal or External Storage in Music folder present on both of the storage devices.
I am doing the same on my Samsung phone and never had any issue because both phones have
Android as operating system.
