Not a bad idea actually, provided the N9 board fits in the upper part of the body. (and I guess there is good chance it does, no sense wasting mechanical design effort when the work has already been done before...)

Well, one way to do it would be to use internals of a BT keyboard and fit the stuff needed in the lower part of the housing.

However, I think it is doable to interface the keyboard directly to N9 board. Now this is not definite information as I do not really know the design details of N9/N950, but as the devices use the same chipsets there is definitely a place in N9 where you can wire up i2c-connected keyboard. Maybe it's even got test pads you could easily solder the leads to, but surely it can be connected directly to input pins if nothing else is available.