Converting File to a corresponding IFIle

If you File refers to a file in your workspace, then yes you can convert it
to an IFile.
Use:
java.io.File file = ...;
Path path = new Path(file.getAbsolutePath());
IFile f = ResourcesPlugin.getWorkspace().getRoot().getFileForLocation(path);

If it's not in the workspace, there is no corresponding IFile so you cannot
convert.