A passive repeater is one of two configurations. Two antennas conencted directly together, or a large flat reflector. The flat reflector has to be very large in terms of wavelength, so you will only see those done at microwave frequencies.
The back to back antennas can be made to work, if there is enough signal strength to work with. You could try a yagi antenna on the roof of the building, aimed at the site you want to improve. Connect the cable from that to another antenna inside the building. Use the highest gain yagi on the roof that you can. It MIGHT work, it might not. To engineer a system like that requires making some measurements and some calculations. Most of the time, you'd find it's not worth the trouble.
Since you're asking here, I can only assume that you're unable to make the measurements and calculations. So try it, and come back and tell us how it worked.
If cost was no object, the proper way to handle this could be a bi-directional amplifier installed in teh building, but that would cost tens of thousands of dollars. The other option would be to add a site to the Mototrbo network you're using and link it.
