Import into ZBrush. Add details.

Import into ZBrush. Add details.

Do this. Low poly is somewhat like very basic models that are usually used to sculpt things in ZBrush. If you're model isn't too sharp, it should be fine.

In some cases, you can get away with simply adding something like Blender's Subsurf modifier. It adds smooth geometry and you can control just how much of it. You can also then apply it and then modify it as you need to. Note that this is similar to what would happen if you put it into ZBrush as suggested. Also, Max and Maya have their versions(TurboSmooth in Max I think) so it should work wherever you go.

Just note that the results will also depend on how the original model is, and just how "low-poly" it is, including just where the the geometry is on the model, as these modifiers use a form of subdivision, so if the geo on the original isn't even, it won't be on the subdivided one either without some extra work.

In some cases, you can get away with simply adding something like Blender's Subsurf modifier. It adds smooth geometry and you can control just how much of it. You can also then apply it and then modify it as you need to. Note that this is similar to what would happen if you put it into ZBrush as suggested. Also, Max and Maya have their versions(TurboSmooth in Max I think) so it should work wherever you go.

Just note that the results will also depend on how the original model is, and just how "low-poly" it is, including just where the the geometry is on the model, as these modifiers use a form of subdivision, so if the geo on the original isn't even, it won't be on the subdivided one either without some extra work.

The other way around is mostly easier, by making different LODs.
You can also take a look at tesselation or geometry shaders (for example to split polys into more polys.

It depends on your needs, for more detail and netter looking models, Hodgman put you in the right direction (modelling).

What is a LOD? I don't recall hearing this term before.

Check here

I'm not sure, but that "smooth" thing in Maya sounds about right. If it is, then what I said probably applies to it.

LODs are level of detail. Basically when a model(let's say a tree, since speedtree does exactly this too) is close up, you see it in high poly glory(let's say 10,000). Then at some distance, you see a lower poly version of it(5,000), and further yet, the tree only has maybe 1,500 polys. Finally, in the distance, it may get converted to a single billboard tree(works well for things that aren't really animated).

Generally if your game will do this, then you may create a super high poly version first in ZBrush. Then create a "normal" version, which you use the ZBrush version to bake normals onto. Then you can lower the polys either directly or using a tool in order to get the lower poly versions.

The issue we currently have is just that you are going the other direction, which is not as common from what I have seen.