It looks like some people in the thread below have figured it out — we actually have this. You can start a chat both ways; the system will automatically recognize if this person is on Skype for Business or on Teams.
Thanks and let me know if you have any questions.
SuphatraJohnathan West commented
We have this problem as well. When I first message a user it gives me the "This user is using SFB..." When I create a chat with the same user a second time, it appears to create it in Teams correctly - not sure why this is the case.