On Magisk "systemless" rooted devices busybox (BB) is located in /sbin/.core/busybox, instead of /system/xbin or /system/bin on SuperSU rooted devices.
Natural way to install BB to /sbin/.core/busybox on Magisk device is 'Busybox for Android NDK' module.
IMO, it is first module one need to install after flashing Magisk.
If device was initially SuperSU rooted, one need uninstall BB using BusyBox app first and after that install 'Busybox for Android NDK' thru Magisk.
It is possible to install BB to /system/xbin on Magisk, but tell me what for?
There's a few things that aren't quite right with your post...
The Busybox you're talking about (in /sbin/.magisk which is the location that has replaced /sbin/.core) is Magisk's internal Busybox used for anything, well Magisk. Module installation and so forth... It's not supposed to be fore anything else. And, Magisk having an internal Busybox means that you do not need to install any other Busybox for Magisk to work.
If you need to have Busybox installed, for whatever reason, you can very well use @osm0sis
Busybox from the Magisk repo, but it won't install to /sbin/.magisk/busybox. It'll install to /system/xbin.
And unless you want to make sure your /system isn't altered (which would also mean you would also have to reflash your devices stock system image) there's no need to uninstall Busybox when switching from SuperSU to Magisk, as long as you have a working setup.
So, to recap, my "no" was simply a reply to your suggestion to install Busybox as a means to fix module installation for that user. There's no need...