To join the Filer to a NT4 Domain you will need to create first the Machine Account on the PDC, to do it use follow command:
smbpasswd -a -m FilerName$
After it you schould be able to join to a Domain without Problems.
In some cases you will need also a working WINS Server it will be helpfully to discover the PDC for the Domain on subneted networks.
EDIT:
if you will get follow error:
[root@PDC-SERVER]# smbpasswd -a -m FilerName$
Failed to add entry for user FilerName$.
[root@PDC-SERVER]#
try first following:
useradd FilerName$ –shell /bin/false