IntelliJ kinda, sorta, does this. You need to open a project with your Java sources. You can then copy/paste expressions, methods, or entire classes in to a .scala file. This converts to equivalent Scala code.
The fidelity of conversion isn’t perfect, and, for this reason, it doesn’t support a bulk conversion yet.
Aside from this, Paul Phillips once started the Scalify project to translate code from Java to Scala (or, potentially, your favourite language), and even improve it in the process! He explains the concept in this video. However this effort was stalled, presumably because he turned his attention to directly contributing to the Scala compiler and standard library.