I don't think you can. But Handcent let's you. It's pretty awesome. I don't know why I never tried it sooner than last night. Just turn off notifications for the stock messaging and you're good to go. Plenty of customization options, and ability to add a signature.
The best part... splitting text past 160. I don't know if its Verizon, Android, Motorola whatever, if you send a message past 160 to non verizon customers they won't get the whole message. My moto q9m did it, so i'm blaming android. Handcent fixes that.
it can pop up incoming sms so you see it w/o unlocking phone.
it can read texts to you and type what you say.