Folders on the homescreen are an artifact, they don't actually exist, so you can't. Just transfer all the apps to the second tab, then drag them, one by one, to the homescreen, merging them into a folder.
One shortcut you
could use is to install
Nova Launcher, tell it to take its data from the existing launcher (probably TouchWiz), then back it up. Transfer the backup file to the second tab, install Nova and restore from the backup (
after you've installed the apps - what's on the homescreen is just shortcuts - if you restore first, you'll have little "Andys" and have to do it all manually anyway,
and delete the Andys]). (I don't think you can tell TouchWiz to set itself up from another launcher - you'll have to keep Nova as the launcher. Which probably won't be a problem. I haven't used TouchWiz since about Eclair - the first thing I do on a new phone is install Nova. Play around in the settings and you'll see why.)