how to install track lighting on preexisting ceiling fan?
That would be "altering the fan". Fans aren't standardized.

What you've done is disassemble the portion of the ceiling fan where the lamp(s) hang down from below the blades.   Those parts of a ceiling fan are not standardized and are totally proprietary to that fan's make and possibly its model.   Nothing will bolt to that, except spare parts for that fan, or other fixtures made by that fan's manufacturer which they have sent to Underwriter's Laboratories to have tested and listed for retrofit.

The reason testing/listing is important, and you shouldn't just jack-rig any old thing, is that fans vibrate.  A lot. The new thing would need to be tested not only to not fall apart itself from the vibration, but also for its attachment point to be durable.  The Ikea item is not tested or listed for that extreme use, and so, you'd have liability problems if your hack was discovered to be at fault for fire or injury. 

Generally the more an item extends out from the fan hub, the more the vibration will affect it, so track lighting is a worst case scenario. 

The "standard" attachment point is the one the fan is attached to.  So you would have to remove and dispose of the fan, and use the Ikea kit instead.  

There are proper ways to extend wiring off a ceiling junction without altering the ceiling itself. But the fan adds a big wrinkle: any such kit involves shifting the fan downward an inch or two.  You must assure it properly supports the dynamic weight of the fan, with vibration factored in.