can i use it for my iphone xs.

Because it is not fast charging. The magsafe charger can only fast charge a phone that is designed to take advantage of the higher capacity of the magsafe charger. When used with your XS it is the same as using a standard Qi 7.5 watt charger. To fast charge the XS you need an 18 watt or higher charger and a cable.

I don’t think so; the MagSafe battery pack is held on by a magnet in the phones that support Magsafe. There are 3rd party wireless power banks; Wirecutter likes the Kenu that uses a “sticky pad” to hold the phone. I have one and it works well.

But keep in mind that a wireless charger is less efficient than a wired charger, so if you are concerned with life of the battery pack a wired pack will give you more reserve energy.

Just put the magsafe charger on a flat surface and put the XS on it. The magsafe charger will then work as a standard 7.5 watt Qi charger.