In Windows 10, if inking and typing personalization is selected under privacy settings, words added to a spelling dictionary will be saved in a file located in the AppData folder.
The file path should be C:\Users\[UserName]\AppData\Roaming\Microsoft\Spelling\en-US
The file named 'default.dic' will contain words the user adds. The file named 'default.exc' lists the words that have been excluded from the dictionary. 'default.acl' is an autocorrect list.
These files work with modern apps and Internet Explorer, but not the standard MS Office suite.