you can work directly on the hfs.ini file, yes, but that would require a restart.
the login information is encoded by base64. you probably need to be a programmer to get this goal.
a little script may be easier.
{.set account|frank|password=theNewOne.}
will do for frank.
it's not trivial, but feasible to loop over {.get|accounts.}
and when {.member of| ThatGroup |user=ThatUser.}
than you set the password with the command reported above.
it clearly requires some programming knowledge

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