Don't know of any extension, but maybe this will help you accomplish what you're trying to another way -- bit convoluted, but at least I was able to jump to a specific point in a convo which is all I wanted.
1) do a search (CMD+f or CTRL+f) on any keywords that you can remember. In the main application window, Teams should return you a list of results having some (very limited) context about places where those words were found.The entries should say something like "John Doe in chat" with a brief contextual snippet which is hopefully enough to at least determine which chat it occurs in.
2) If you find a convo exerpt that looks like the one you were after, go to that chat by clicking on it. In this case it'd be your chat with John Doe.
3) From inside the relevant chat, do the find shortcut again (CMD+f or CTRL+f). Teams will now use the context of the current chat to perform the search again with whatever keywords you want -- the same as originally searched or new ones. You'll notice that now the search bar has a "/find" prefix with the name and avatar of the person in the chat you're searching on
4) Teams will now return you a list of results in the side panel with corresponding dates, and a similar snippet, but limited just to the current chat context.
5) From there you can go to the exact point in the conversation where the keywords occurred and after clicking, you'll notice that the point of interest will be briefly highlighted to call your attention to it immediately.
Hope that helps
