It's actually very simple to go from fibermesh, generated and sculpted in ZBrush, to an nHair system in Maya by using exported curves as an intermediary.

First, you generate the Fibermesh through the Fibermesh submenu under Tools. You can use masking to specify what part of the model you want the fibermesh to apply to. The particularities of the settings you choose depend on what sort of fur or hair you're trying to achieve. Generally, you want 100% coverage, but the number of fibers influences the number of curves generated. More on that in a bit, but you'll have to experiment to see what is the right setting for what you're exporting. For example, full-body fur (which is actually rather difficult to do with dynamic nHairs) would require more fibers than just a tail.

Next, you sculpt it with the groom tools and other sculpting brushes. In particular, I needed to use the FormSoft brush to resize the different parts of the fibermesh so they weren't quite so uniform. At one point I made extensive use of the swirl, twist, and similar grooming tools to create a spiky-clumpy look, but it didn't translate well to nHair. This is because what you are basically doing is shaping curves that the nHair will generate from. The exact form of your fibermesh created here will NOT translate over to the nHair. So don't spend too much time getting exacting details down.

Once you've finished with the fibermesh, you need to export it as curves (which is found on in the fibermesh tool submenu.) Now because ZBrush has (IMHO) a terrible tendency towards the bizarre decision of reusing components without creating a separate UI element for clarity, you have to set the number curves you export based on the preview setting. This is found right above 'Export as Curves' on the same submenu.

This is where things can be a bit complicated. You don't get a 1-1 fibermesh to nHair conversion, so you've got to make sure you've sculpted and choosen your fibermesh settings accordingly. Expect to have to restart a couple times as you get used to it.

My initial fibermesh tail is found on the lower row of images. I used good-looking fibermesh settings, but this resulted in far too many curves. On the lower left is a preview pre-vis setting of 1 which would export nearly 6,000 curves. On the lower right is pre-vis at 100 (the highest setting) and it would have exported over 300,000 curves! Both of these are far too many. The 100 setting tended to crash Maya with even the most basic pose changes. The 1 setting was a bit more stable, but because of how the curves influence the nHair, actually looked pretty bad... and it crashed a lot too.

The revised fibermesh tail is found on the upper row of images. This used a much lower number of fibers, resulting in significantly cruder shape and far fewer curves. The upper left pre-vis setting of 9 results in enough curves that it doesn't even warn me when trying to export them, so I don't actually know how many there were. The pre-vis setting 100 on the upper right would result in ~9,000 curves and was unsuitable.

Once the curves are imported into Maya, it's simply a matter of selecting the curves, the model, and then using the "Make selected curves dynamic" option under the nHair menu. I generally use the options to make them exact, snap and attach to the surface, collide with the mesh, and output to NURBS and paint strokes. However, I also found afterwards that I generally had to select the Hair System and make it "Add Paint Effects to selected nHair" before it created the Paint Effect.

If you don't want to make changes to the guide curves of the nHair afterwards, you don't have to generate the NURBS or you can also delete. At the very least, hide them because they make the scene seriously slow! The paint effects do too and can also be hidden when you're not working on them.

That's all there is to creating nHair from Fibermesh! More importantly, this also works for any ol' curves you create. ZBrush has proven useful to me for dealing with mass number of curves. However, I think hand-created and modified (or maybe programmatically designed) curves would ultimately be more desirable for more detailed stuff.