Can I copy OSX10.8.5 to an external drive?

Hi there, I still have OSX 10.8.5 on my internal drive but mostly boot up from an external drive with Yosemite 10.10.5. The only reason I’ve kept 10.8.5 is because of DVD Studio Pro and Compressor, because I still need to author DVD’s and High Sierra won’t allow me to open those apps, and Sierra on an external drive isn’t recognised as a bootable startup. So, am I able to copy my current10.8.5 to an external drive, then update to Sierra or later, and then have the option of rebooting from 10.8.5 on the external drive when I need to, or will Sierra not recognise it. Many thanks.