You can't yet I am afraid, the author said he is working on a Windows Phone 7 version... No sign yet and the comments about how long other platforms have been waiting for updates is discouraging. Could be a while yet.
There is also ShapeWriter, but I don't know if they are doing a wp7 version.
I understand that you might prefer T9, so many people can do it with their eyes closed (and walking or on a bus, for sure). The standard keyboard tries to predict what you are trying to type, but it gets it wrong too often for me and the sheer hassle of having to concentrate on pointing exactly at the key you want and then missing and having to hit backspace - and missing that! Is a PITA.
Swype lets you be inaccurate and that, combined with the one "press" per WORD is how you can be so fast. Plus it's correction techniques are much faster than the standard keyboards.
I am sure there must be a "hook" in for other input methods.
Now I have SatNav, it's my biggest miss on WP7. That and the lack of hackability... And no iPlayer... And no copy and paste... And no multitasking... No Google maps (streetview)... Hmmm, can't wait for this Jan update!