Yes, it is possible.
But one user only has one set of default applications.
Sure, you can create a new user and ssh to it (ssh user2@localhost) and make the default shell of that user bash (or other) but that is probably not what you meant.
Then the rub lies on deciding what will trigger the use of one shell or another.
You can just type
bash -l at the command prompt and start working with it.
Later (when/if needed) you can make this easier.
-l option will ensure a clean set of environment variables and the use of default configuration files like
The easiest way is to use a different console application (gnome-terminal, konsole, xterm, etc) and configure it to start the shell you want on start. Assuming that you use konsole in KDE, and i3-sensible-terminal is the default for i3, you can configure gnome-terminal to load bash on start and use gnome-terminal as your terminal. Go to Edit --> Profile Preferences --> Command --> Run a custom command instead of my shell --> Custom command -->
Equivalent settings could be done with most other shell terminals.
Then it is just a matter of starting the related terminal to get either zsh or bash in any of the DM.
If you have (or could install)
wmctrl, then, this line will tell you which DM is currently running:
From that, you can decide which profile to load in konsole:
case $(wmctrl -m| grep 'Name:') in
Name: i3) konsole --profile i3 ;;
Name: kde) konsole --profile kde ;;
Of course, you need to edit the configuration for each profile like font size, colors, etc.