This information applies to version(s): 2018, 2, 3, 4, 5
Nik Collection is not currently compatible with RAW files. To use Nik Collection with images processed through DxO PureRAW, the file will need to be exported as a JPEG. In this case, the image can be exported directly from DxO PureRAW to Nik Collection software.
The workflow recommended by DxO is to start by optimizing your images with DxO PureRAW as DNG files, then export them to a host application that supports DNG files, and invoke the Nik Collection plugins from there.