Currenty, this can be done only if you turn on HTML code on your forums. Although I do not recommend that for security reasons.
Or you could MOD the inc_func_common.asp to allow the user to use [table] [tr] [td] as forum code then replace them with the proper HTML code. But you need to know some programming to be able to do this.
In addition to what iane87 mentioned, there was a MOD that someone started for adding Table Tags (more likely than not its in the archive) and I'm not sure if it was ever finished or not. The problem is that if someone does not properly enter the table codes (via the MOD or by allowing HTML) it will jack with the layout.
You'll be hard pressed to find a forum package that allows for tables in some form or another just because it takes a fair amount of overhead to make sure that someone didn't screw it up.