It would probably be good, if we knew in advance what your ultimate goal is.
If your goal is to have something bootable, then that answer is probably..... no.
In most instances a bootable USB device requires preparation in advance for that purpose and if you search on line you'll find that most prep utilities will wipe the device to get it ready.
If all you want is the ISO file to perform an in-place upgrade from, then you can put that file in and on anything. You can place it in a folder on the USB drive.
Once you have the ISO file then you can mount it from within Windows 8 or Windows 10 and perform the upgrade from there by simply evoking the setup.exe application.
If you have Windows 7 you will need an additional tool to mount the ISO. Microsoft provides one that they say will work although I've never tried it.
http://www.microsoft.com/en-us/download/details.aspx?id=38780
Or you can use something like 7zip to extract the contents of the ISO file, where ever you want to place it on the USB drive and again run the setup.exe application from there.
You can google that process or simply right click the ISO and chose extract here.
Finally, if the contents of the USB drive is critical to you then I would strongly suggest copying them off to an alternate location in advance of using the device for anything else other than storage.