It is possible to install bash by
# pkg install -y bash
bash requires fdescfs(5) mounted on /dev/fd
If you have not done it yet, please do the following:
mount -t fdescfs fdesc /dev/fd
To make it permanent, you need the following lines in /etc/fstab:
fdesc /dev/fd fdescfs rw 0 0
However, I immediately changed root's default shell as sh, as I csh is just too different from bash. For instance, one needs to use setenv in place of export in csh.
To change default shell to sh, run
# chsh -s /bin/sh
and change the default shell from /bin/csh to /bin/sh
Note: do not change root's default shell as anything other than those included in the base distribution. Read here for more details.