Unlike the FM radio, there is no way to connect an external antenna to the N900. Not even through the USB jack or the headphone jack. The FCC may be the reason for that. The wavelength of the FM band makes an internal antenna next to being useless. One possibility is to make a passive antenna repeater that the N900 can be placed next to.

Maybe duct taping a coat hanger to the phone will work. A little bit tin foil can be used for fine tuning.