Hi Hubert,
You can export your tracks from DD as CSV files, which can then import into RM, but if you want to use the sonar viewer you are going to have to import the original log files.
Shorelines can be imported as KML or Shapefiles, or taken from any track within the workspace.
Bear in mind that RM doesn't run on a "one big database" model in quite the same way as DD. Better to import just the tracks you need for particular projects into each workspace. Each workspace is a separate database.
There is no "bulk import" as such, but you can select multiple files of any combination of different types and import them together. If you are importing files in a nested directory structure, then use the search feature in the open file dialog as detailed in
this post. In principle, you could search for all files on your drive and import them this way - might take a while tho :)
cheers,
Matt