The path from SBT to Scala-IDE is well described in many places:
No, you cannot do this. The way to do it is as you described. Then, whenever you make changes to build.sbt (e.g., new jar dependency), rerun the eclipse
command from sbt and refresh the project in Eclipse so that the newly generated files are reloaded.
As a seasoned Eclipse user, I wondered the same thing. Amazed that no one seemed to be going that direction, I decided to roll my own project.
https://github.com/scalastuff/esbt
Install plugin, create or modify build.sbt and dependencies are fetched, project files updated.
It works for me, I use it every day. But it's not perfect or finished, and I'm not putting in a lot of effort in it at the moment. But do feel free to contribute!