Many occupancy sensors allow you to adjust the amount of time that they can't see you before they shut the lights off. I'm not sure what model you have, so I can't instruct you exactly how to adjust, but setting the timeout time to something longer would probably save you the trouble of building something to circumvent it.
For example, a panel on this Leviton switch pops off to reveal some adjustment controls.
PDF instructions
If your office is managed by your company or a building manager, it may be worth asking the maintenance guy.
Occupancy sensors, timers, dimmers, and other "smart" switches often are required to be independently powered. If you look at this diagram from the devices documentation (PDF), you'll see that there are three ways this requirement is achieved.
Neutral Wire Required
The first method, is to simply require a neural wire. In this configuration, the device draws power using the ungrounded (hot) conductor and grounded (neutral) conductor. It also has a separate switched conductor, that it uses to control the load.
This setup would be wired like this...
Minimum Load Required
This method draws power using the ungrounded (hot) conductor, and the switched conductor. So the device is actually in line with the load.
This setup is wired like this...
Ground Wire Required
This method draws power using the ungrounded (hot) conductor, and the grounding conductor. It's wired similar to the Neutral Required devices, however, it uses the grounding conductor instead of the grounded conductor. This means that there will be a small amount of current on the grounding conductor, and that the grounding conductor is required for the device to operate.
This setup would be wired like this...
Notice there's a bare, and green wire connected to ground in this diagram.
tl;dr
Your device
The device you're using (MS-OPS5M-XX) requires a ground to operate, according to the documentation.
Solutions
Install grounding conductors
One solution, would be to install a grounding conductor with this circuit. This will likely require quite a bit of work, and might be quite costly.
Install a grounded conductor
It may be possible to extend a grounded (neutral) conductor from the light to the switch box, which could then be used to power the device. In this case you'd have to purchase a different device (one that requires a neutral rather than a ground).
Best Answer
Put the sensor in the space you are trying to light, and suddenly this "common need" becomes "not a need at all" which is why you can't find them, since that's how it's done, when done conventionally.
Either move the switch into the room or get a remote sensor switch and put the sensor in the room.