Convert text message to email App?

I don't understand what you mean by convert text messages to e-mail messages. If you want to send an outgoing message to someone and you don't want to use SMS/MMS, just send an e-mail. As for incoming messages, there is nothing you can do to magically transform them into e-mail messages. If you don't have them completely blocked by your carrier, they will come through as SMS/MMS.

You can use Apple's iMessage service to text. This goes towards your data plan.

Look for something like this in the App Store:

http://itunes.apple.com/us/app/xfinity-mobile/id320788270?mt=8

Google voice might be something close to what you're looking for as well.

Or iPad or iPod touch or now Mac's running message beta...requires Lion.

You can send a text via email with most carriers. For example, if you send an email to <mobilenumber>@vtext.com, that email will show up as a text on the Verzion Wireless customer's phone. AT&T and Sprint have similar addressing schemes.

You don't send an e-mail out of iMessage. You send an email from mail.

I think you need to clarify a little bit just what it is you're trying to accomplish.