MFractor includes a localisation wizard that can move strings in a C# file into a RESX file and replace the original string value with a resource lookup.
Open the localisation wizard by right-clicking on any string literal and selecting Replace with resx resource lookup.
Once opened, you can choose the .resx file to place that string into, enter a key for the new resource and then selecting replace. Alternatively, you can apply the localisation action by press the Return key.
If the string value already exists in the .resx file that you have chosen, MFractor warns you that it's a duplicate with a yellow exclamation icon. If you'd like to use the existing key for the string value, you can press Shift+Return.