Ah, that's easy. Use Dolphin's memory card manager (it's in the tools menu)
"Browse" and select the .raw file, select the correct save in the list and press "Export", or just press "Export All"
If you ever want to do the reverse, it's also simple with the memory card manager:
(Optional: "Browse" for the .raw file you want to import the save into first, otherwise you can use Dolphin's default one) "Import" and select the .gci file, it should be imported into slot A. Check the path where the slot A .raw file is located, copy this file to the correct location.