I don't think that's possible as whatsapp has a lock down on its messaging protocol. Mightytext developers can't achieve what you're requesting without specific permission and major help and special care from whatsapp developers. You're better off asking whatsapp developers to release an API .. only then you can ask mightytext developers for an integration.
For all the wonderful voters on this feature, ask yourself the following question.
Would you like to manage yet another application on your desktop specifically for sending text messages, or would you rather integrate text messaging into your already existing instant messaging applications such as skype, yahoo messanger, pidgin, adium, ..etc ?
If you prefer the former, then keep your vote here and don't change anythinng.
If you prefer the latter, please move your votes towards a suggestion that better illustrates that desire for integration. http://help.mightytext.net/forums/123495-general/suggestions/3416105-mightytext-as-a-protocol-api-
Before voting here, please refer to this : http://help.mightytext.net/forums/123495-general/suggestions/3416105-mightytext-as-a-protocol-api-
I don't think iMessage integrations is doable at the time of this writing as apple hasn't released an API for it (and probably never will). It's a selling point for apple that their messages are available across all their devices. This notion is effective in marketing. Say for example you wanted to buy a tablet when you already have an iphone. You'd be more likely to get an ipad for the out of the box integration you'd get among those devices. It's a monopoly. Apple is isolating itself in a bubble and that's what drives their sales up. If you get an iphone, you'll get a mac and an ipad at some point in your life.
As for other IMs, it shouldn't be the responsibility of Mightytext to connect with all other protocols. That would take too much of the developers time and effort only to replicate solutions that are already out there for such integration (ebuddy, IM+, ichat, empathy, pidgin, adium, ..etc) However, it would be a great advantage to Mightytext if they focus their efforts on solidifying the backend and publishing an API for other developers to adopt in their respective projects. Currently, there is no IM application (that I know of) that can send SMS to your contacts using YOUR phone number. There are currently apps that send SMS but they generate random source numbers which confuses the receiver on the other end. The use of your own personal already known phone number is a great advantage that Mightytext has over using the many other SMS solutions out that. The developers should not lose sight of their unique selling point and should focus on seizing the opportunity while it's still untouched. IMHO that's the niche market Mightytext would excel at.
Don't recreate solutions that already existant. Leave integration to IM apps developers. Develop the Mightytext protocol as an encapsulation of SMS messaging and publish an API. That's the route of greatest success.