I'm afraid the 10 rpm motor won't transfer any torque to the 60 rpm shaft through the freewheel. If the shaft is already spinning at 60 rpm from some other source then what are you trying to accomplish with the 10 rpm motor? You already have a 1:1.5 ratio, so you need another 1:4 (or higher) ratio between the 10 rpm motor output and the pinion gear if you need to maintain the 60 rpm.
If it is okay for the shaft to slow down to 15 rpm before the motor starts applying torque, then your concept will work. The shaft starts at 60 rpm, slows down to 15 rpm, then will maintain 15 rpm by the motor. Without the freewheel, the motor would act like a brake, but with the freewheel the shaft will slow down as if the motor was not there (or nearly so) until the point it would go slower than the motor which the freewheel will prevent. The freewheel is a one-way-clutch just like on a bicycle with handbrakes. Going down a hill, you can't slow the bicycle wheel down with the pedals, but once the wheel is going slow enough for you to keep up with the pedals, you can apply torque and keep it going at the slower speed.