What you are asking for is a very complicated setup.
squid can authnicate to a lot of different things including pam and ldap
im thinking that the machine that squid is running on is using ncsa htpasswd type authenication cause if it was using pam you could just use the same passwd file from the other machine instead of keeping two seperate user lists.
but i cant be sure.
When a user authenicates linux has a set of files that tells it what to authenicate by, first it will ask pam if that user name exist there then it will go to ldap and authenicate there.
there is a package called openldap that you can install and configure that to have the hardware firewall authenicate to.
But openldap is not very simple nor is it straightforward.
and there are tutorials on how to setup openldap for a domain controller you would have to do the same thing and then addusers with one of the gui clients available like gq or something like it.
And then have that hardware firewall authenicate against it.
I have never used suse but i hear that suse has a very good ldap configuration tool so you might want to look into that.
in fact i think if i were you i'd go with radius rather than ldap.
there is a program called freeradius that can auth using pam so no need to create additional users.