If you only want it to last until the next reboot, then get into the shell prompt and enter:
root@zeroshell root> chsh -s /bin/bash
I do this when I want to mount the Zeroshell’s disk on my Macintosh’s desktop using Macfuse and Macfusion and the ssh filesystem. Makes writing patch files to the box a lot easier.
If you want to make it permanent, you could put the same command into either pre-boot or post-boot scripts.
root@zeroshell root> chsh -help
chsh: invalid option — ‘h’
Usage: chsh [-s shell] [name]