While the linked thread does tell you how to create extra folder for IMAP and Exchange type accounts, those folders are stored on the server, not locally (on the iPad).
I know of no way to create a local folder in Mail on the iPad.
Possible solutions:
There are third party apps that will allow local folders. IbisMail is the only one the comes to mind, but I know there are others. That's not a recommendation, btw. I haven't any experience with any third party mail apps.
Another workaround is to forward emails to another service that in turn will sync back to the iPad. I use Evernote this way. I can forward an email to my Evernote account, even specifying a notebook (by adding @notebookname to the end of the subject line). You need a paid, premium account to keep notebooks synced with the iPad.
There are other possibilities, but I haven't really explored them since I'm happy with my Evernote solution. At $5 a monty (cheaper if you pay by the year) I'm very happy with my premium Evernote account and having my notes (and other stuff) available just about everywhere. If you're not familiar with Evernote, here is a thread where I gush on and on about it's features and how I use it. Oh, and others talk about it too.
http://www.ipadforums.net/ipad-apps/105051-evernote-how-do-you-use-tips-tricks.html
Here is what an email forwarded to Evernote looks like. Note that the from and too addresses are clickable, so you can still click on it and reply to the sender (though you will have to copy&paste manually if you want part of the body sent.
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<img src="http://www.ipadforums.net/attachment.php?attachmentid=46706"/>