In Windows, CustomRoaming.dic is stored at C:\Users[YourActualUserName]\AppData\Roaming\Microsoft\Office\16.0[7DigitRandomHexNumber]\Proofing
In Word for Mac, the custom dictionary is stored in the hidden user Library folder. To open this:
- Hold down the Option key while clicking on the Go menu in the macOS menu bar.
- Click on Library. Your user Library folder opens.
- Navigate to ~/Library/Group Containers/UBF8T346G9.Office.
- The custom dictionary is a text file called Custom Dictionary.
I tested, and you can delete the default Custom Dictionary file, then copy CustomRoaming.dic into the same location and rename it Custom Dictionary (no file ending). It's working here.