Hi Medsec,
I had a look myself before trying the web, because I know quite a lot about Word and it's custom dictionaries. However my setup is not quite standard as I have had multiple versions of office on it.
There is an article
here which discusses the simple bits.
My one in the Uproof folder had a list of 3 words in it.
However, in Windows Explorer (windows7, File explorer in 10), if you back out of the Uproof folder and open the Office folder in the same tree, you will find at least one set of numbers. I have 15.0 and 16.0. In each of those folders are two additional folders consisting of 8 letters and numbers. Inside that folder is a proofing folder and inside the proofing folder is a RoamingCustom.dic file. The one mentioned in the picture in the link I sent you. Across those 4 roaming custom dictionary are all the words I have saved.
The easiest way, I find, to deal with this, is to open the custom.dic file in Word. Then open each RoamingCustom.dic file in turn and copy the words from it into the custom.dic.
You can then follow the instructions in the link and point it at your "newly enlarged" custom.dic file.
As an example, my custom.dic file is in
C:\Users\[profilename]\AppData\Roaming\Microsoft\UProof where [profilename] is the name of the account I log in with.
My roaming custom dictionaries are in
C:\Users\[profilename]\AppData\Roaming\Microsoft\Office\15.0\6f6cf5d8\Proofing
C:\Users\[profilename]\AppData\Roaming\Microsoft\Office\15.0\261b46f3\Proofing
C:\Users\[profilename]\AppData\Roaming\Microsoft\Office\16.0\9eed9f0e\Proofing
C:\Users\[profilename]\AppData\Roaming\Microsoft\Office\16.0\b92b0952\Proofing
Respectively.
If your original word is older, then
this link might be better and you may not need to worry about the roaming dictionary.
Let us know if you have any issues. It should be simple but, as usual with Microsoft, it is not.