I'm not totally sure I understand what you're saying, but I can give a stab and you can let me know. (This is for iOS)
On your iDevice, Open the settings app, and scroll down to messages (or facetime to use it for that). Click on that, and then scroll down to Send & Receive. Click on that, and click add e-mail. Note that on iOS, you can only add e-mails. There's a way around it for some carriers.
To derive the e-mail address, use the first part of the e-mail address as the your cell phone number you want to add, and following the "@" symbol, you type the carrier's domain name. For example, texts to AT&T customers should be sent to "txt.att.net" – so if you have service with AT&T and your cell phone number is 1-222-222-2222, you would use the e-mail address "[email protected]
All of this is only theory- I've not done it. But, I do use my e-mail on my device in this manner, and it works. So I'd think it would.
The last line of your OP is the reason I think that I might be barking up the wrong tree, but I figured I'd post it just in case I wasn't.