Is there any addon out there that enables me to get a new "target" like thingy for mobs that are marked with skull, star etc ?
I don't think so, but I could be wrong. Targeting macros do the job pretty well though. Especially useful on fights like Ryolith where you need to switch targets quickly, keep one on focus, and work on sparks.
Targetting macros if you want to do that or focus macro's you can't detect if a target has a mark...
It is not possible to target by raid mark with an addon or macro.