There is no "bash" though I assume you really just mean mean "shell". On pfSense it defaults to tcsh/csh.
Don't try to add users from the command line (and if you lookup how to do it on FreeBSD you'd see there are many ways to do it, especially the pw command). Just add them from the GUI.
Also I wouldn't expect that user to stick around indefinitely, especially if you're using an embedded install.
Perhaps if you explained what you are really trying to accomplish you might get some more constructive feedback.