Gboard (Google Keyboard) has a very simple way to add new words to it.
Open the Gboard app, go to ‘dictionary’ > ‘personal dictionary’ > (Japanese) > plus (+) symbol on the top right. Now, type the desired word, and add a shortcut, that way every time you type the shortcut, it will give you the option to switch to the desired word.
For example, if I want to type the word
Привет in Russian, but I don’t want to switch my keyboard from English to Russian just to type that word, so I made a shortcut that every time I write the letter P the word will show up in the suggestion bar. See screenshot below