I''m writing the design doc for a new game that I''ll write in VB with C dll''s. It will consist of several environments and a number of "actors". I''m wondering if it is possible to use a level editor, such as for Half-Life or Unreal, to manufacture the environments. And if so, I''m not clear on how those levels are imported into my code. Anyone tried that? Thanx!

Those editors output to a common format which you have to understand to use. I would Google the string "MAP Half-Life format" to get information on it.

Writing the loader is up to you, and how you use that information is engine-specific as well.