TTL oscillators do not require any additional components.

Connect it to OSC1/CLKIN on the 1825 and use ECH mode ...
--> ECH: External Clock, High-Power mode (4-32 MHz): device clock supplied to CLKIN pin

That frees up the OSC2/CLKOUT pin for general I/O.

On the 629 and 683 there is only one EC mode, but it works the same as ECH on the 1825.