If you mean can the EX5 sound like the motif, then no. It will still be the same ol' EX5 you know and love.

However, when connected to the motif rack via MIDI, will control what the motif does.
Most MIDI devices default to MIDI channel 1, so hopefully to will not need 'mess' with any settings to begin with to get going.
The audio outs will need to be connected to your keyboard amp (if your using one), or your mixer (if your using one).

You can 'layer' patches from the EX5, and motif rack, providing both audio outs are connected to your amp and / or mixer.

Connecting the MIDI out of the motif to the MIDI in of the EX5 will do very little...

So to sum up:
Keyboard(EX5) MIDI out ----> MIDI in of motif

MIDI is control a control protocol : no audio is carried down the cables.
Any MIDI device, be it Roland, Korg, or Yamaha will communicate with another MIDI device.