Is it possible to add fly? This can be much useful.

Technically, it was in the legacy Xray mod, so maybe if we have the time. If you really need this feature, you can use the mods listed by loonybtard

Technically, this is possible. In fact, it is not necessary. Its xray mod, not pack of mods. IMHO, if you need several tools of this kind, it is better to use ready-made clients like Aristois, Impact, Kami Blue, Future and etc

There are other mods that let you fly such as https://www.curseforge.com/minecraft/mc-mods/fly-mod-3d. There are also mods that let you create things to fly on.

严格来说，它是在传统的Xray mod中，所以如果我们有时间的话。如果你真的需要这个功能，你可以使用loonybtard列出的mods
I need is very much, thank you brother！