You can text message easy enough. When you add in their mobile number to the address book, you will see a new option to SMS [user] when you click on their name. Just type in what you want them to see and they will receive it on their phone.
Look in the aftermarket forum for options for IM, there are quite a few decent ones out there such as Verichat, WebMessenger, IM+ and others. Most of them will cost you a few dollars (maybe the price of a good haircut), but are well worth it.