How can I install ceiling mounted recessed occupancy sensors?
There are switches that have motion sensors in them.

There are also Light fixtures that have sensors in them, you would need all new fixtures.
If by spot lights you mean recessed lights this may not work.

We had the switch type in a rental once. I may put them in a hall but never in a main room as every few minutes you had to wave your arms to turn the lights back on. I dont remember how far the sensor could see.

One of these will be the easiest way to add motion sensors, if none of these suits you more info will need to be given about the wiring you wish to change i think.

Do you have a link to the type of device you want to install?