So I've been wanting to put a windows 10 ISO on my flash drive, but I have WinPe by strelec already on it. I don't know how I can a windows installer on top of it and I don't even know if that's even possible at all... Please send help I'm a newbie when it comes to software
Unfortunately no, it is not possible to have multiple bootable OSes if something is already present, (at least to the extent of my knowledge) but if you'd like to dualboot on a clean USB drive, (erase your existing one, even) there exist several tools that can do that, such as Easyboot, syslinux, or even grub4dos. Some other helpful resources can also be found at Reboot.pro. However, if you are currently running Windows, I recommend WinSetupFromUSB as an easy way to get a multiboot USB drive going.