You could try following
this guide for adding persistence to a live usb. It's for Kali but it should work for you with perhaps a little modification.
Edit: Or maybe not, I don't know if Slack will give the needed option in the installer menu
Edit 2: It looks like this should be possible with Slack: "The USB version is “persistent” - meaning that the OS stores your updates on the USB stick. The CD/DVD versions (and the USB stick if you configure it accordingly) operate without persistence, which means that all the changes you make to the OS are lost when you reboot. "
Have a look
here