There are some inductive antenna couplers. However. they can lose 4-8 dB, so you need an antenna with bigger (real, not advertised) gain than that, which rules out all the desk-stand omnidirectionals. So, a directional antenna, mounted on the outside of the building, carefully aligned towards the nearest celltower. As the distance between antenna and device will be great, you'll need some lowloss cable, which will be thick.

At this point you'll have a frankenstein n900 with a big coupler attached and cable that looks like somethng you've stolen from the electrical grid.

Maybe a better idea would be to get a WLAN AP that has USB ports and support for 3g dongles, and a 3g dongle with proper antenna port. This would reduce the size of the antenna needed, and you could surf the web on your n900 wirelessly over wlan.