Hi,
I may misunderstand a few of the things you say, but let me propose some ways forward that may or may not be of use to you. (there are at least a few ways to do something like this.
It sounds to me like you are planning to import the landscape as an object and simply color it. You can do that, but I don't recommend it as the best way forward. because you will inevitably run into issues when adding displacements to the object. Also, you cannot populate on an object yet, so forget about it.
But if you only want to add color, simply import the object, set its 0,0,0 to the the position on the tg planet you want (ideally also at the planets 0,0,0) Now go to the object node you just imported and open it, this is where you will add your nodes for color detail.
If you want to do it this way, then say so and more help can be given for the how too and such.
Adding bump is easy as pie, but real displacement on an obj is more problematic... It depends on how detailed the model is in the first place. The more quads/detail on the object you have will make adding displacement better. But it also matters if you want an animation or a still.
A still will give you more wiggle room. For example, you can render out RTO on, and another with RTO off, and post in Photoshop. But you would not want to do this at 30FPS.
RTO on will give you bump, RTO off will give you real Displacement.
OR
1) get a satellite data map for free from the us geo department of the location. And use this data to build a real terrain of the place. This will be probably the simplest, best way to do a real US location in terragen... If you can find your way around TG.
2) Convert your model into a vector displacement map in mudbox. This option will work but it will be some trouble, since first you will have to make a plane with the same number of verts as the model, and then attach the model to the plane, as if you modeled the landscape up from the plane object. Maya can automate some of that, but you likely have a ton of clean up. Then send to mud do some work there, and save a vector displacement map
Really, it is hard because you are coming to terragen after starting the project, rather than building the model with terragen in mind in the first place. But there are a lot of things that you can do that will look good. It would help a lot to show a screen capture of the model in Maya, so we know exactly what we are working with.
Other users may understand you better and be able to instruct you sight unseen, but thats a lot harder for me.
Options 1 and 2 are the only ways listed here that you can populate on the object.
