Yes, you can absolutely do that. You would use the KMLPropertySetter transformer:
Because you can add HTML content there are a lot of things you can do with this. You can add links to images, and you can also link to content such as YouTube videos, tables, documents... almost anything.
If it helps you can find some learning materials for FME and KML on the Safe Software website. There is a tutorial and a training document you can work through.
There is also a set of webinars available, one of which is "How to Prepare Data for Compelling Displays in Google Earth" and includes (I think) the YouTube embedding example.
(NB: I do work for Safe Software)