Possible but no margin for any error.
Singapore airline arrives at B42. FRA-Nice leaves probably somewhere from A. You can use the B to A underground tunnel. . You need to go through passport control / immigration at Terminal B and security. The elevators are next to B19 and you arrive next to at A15.
Passport control is on level 3 after that you go down to level 2
Level 3
Level 2Passenger tunnel A/B
If you are changing between piers A and B in Terminal 1, you can use the passenger tunnel. On the moving walkways, you can enjoy the gigantic work of art by Martin Liebscher.