Not 100% sure what you mean, but you can have multiple terrains in Unity with each one having a different heightmap, which can be used to make extremely large areas.
I’ve been able to import L3DT terrain heightmaps into Unity without much of a problem.
The only hitch are the alpha/splat maps that L3DT generates for blended terrains. For some strange reason Unity forgot to expose them in the inspector. You can see it, but you just can’t apparently replace it via drag and drop. You have to do it through an editor script floating around on the forums.
L3DT does generate some really stunning terrains. It’d be nice if there was a 1-button Unity terrain export like there is for TGEA.
TGEA allows you to have a terrain with one incredibly massive texture laid down onto it. Of course those non-blended terrains end up being like half the size of a CD-ROM but they sure look good.