lgexalter1 so far I've only had success doing it on the phone itself with magiskboot, which takes a bit of time to repack. I work out of /data/local/tmp:
/data/data/com.topjohnwu.magisk/lib/libmagiskboot.so unpack boot.img
/data/data/com.topjohnwu.magisk/lib/libmagiskboot.so cpio ramdisk.cpio 'add 0700 overlay.d/custom.rc custom.rc'
/data/data/com.topjohnwu.magisk/lib/libmagiskboot.so repack boot.img neutron.img
if you're messing with super I did get it open using lpunpack. Not sure where that came from though...