Can i paint other materials with this and still make it look like wood?

No it will just look streaky due to the transparency

I need to stain cherry colored (already stained by unknown to me paint type) drapery rod rings into dark brown. Would this paint work?...

The varathane product shown is a wood stain. Assuming the drapery rings are wooden, it would be difficult to get a good stain effect even when well sanded. Would suggest a gel stain, which is more paint like or just get a walnut paint. This is from my experience in restaining wood, others may have better advice. Good luck.

Can this be used over woodfiller ?

Yes! I used filler and went over the top without any wipe off. Looks perfect and only took half a day to dry (sooner if you follow the instructions to wipe but I liked the stronger pigment and lighter grain look).