can anyone please tell me a quick way to be able to let these guys forward the calls to an answering service at night using an old meridian system?

They just got VOIP service from att and they do not offer CF on these lines.

Can I just create a CF button and use a 9 in the dial string? Is it that easy?

You are missing a lot of info.

What system is it? Norstar? BCM? Option?

What version software are you running?

What type of lines are on the system? POTS? PRI?