if you're new to zbrush you should take a look at the video tutorials
there are some free ones on the zclassroom and they are pretty short (google zclassroom, it's one of the first links), there are basics there too... you should take a look because zbrush isn't like a regular program.

taking the time to learn how to use your software will save you time in future

also, you could just make a bump map in photoshop or whatever painting program you have. just make a greyscale image. you may want to change the thing from NURBS to polygons, NURBS come with their own UVs and by the looks of things you have a giant seam in the texture

also, i suggest you look at this:
https://www.simply3dworld.com/movie_p...tml?tut_id=156
it's free


ok, so maybe you might not find what im talking about when i mentioned zclassroom..
zclassroom:
https://www.pixologic.com/zclassroom/homeroom/
click: "getting started wtih zbrush"
you you are still going with coldwaves suggestion of normal maps then pay attenttion to some of the things in "essentials", and everything in "normal maps for maya" (click "chapters" near the bottom to see these options)