It should be possible to add users automatically, but adding machine accounts automatically is a bit insecure, isn't it?
In the smb.conf file you can add a line for the users:
add user script = /usr/sbin/useradd -d /dev/null -g machines -s /bin/false %u
(or something like that to match your needs
Maybe you can write a little script to add all the machine accounts, but I wouldn't know where to get their names from....