When trying to use the automeasure function on our 86142A , don`t find an input signal. is the 86142A a suitable instrument to use for testing multimode sources ?

Wherever the transition from 62.5um to 9um core is made, there will normally be high loss. The amount of loss depends on the beam profile (modal distribution) in the MMF. So the 86142A is not very suitable. There was a multimode model OSA as well, with MMF input.