Place the Pad sound in PART 1 and the Grand Piano in PART 2 of a SONG or PATTERN [MIXING] setup.
Press [1] to select the Pad sound.
Play the key C# and step on the sustain pedal
Press [2] to select the Piano sound, play the piano.

The C# held by the sustain pedal will continue as long as you continue to hold the pedal down, when you release the pedal the Pad sound will continue to its "AEG RELEASE (time)" parameter and disappear, this, totally independent of you playing the Piano sound from PART 2.

OK. I tried that but I'm not able to use the sustain pedal for the Grand Piano. Maybe i could use a midi controller with a 'hold' feature for the pad part? How would i setup a midi controller to control a specific part of a song or pattern?

Again you would use a SONG or PATTERN MIXING setup - this is where each of the 16 PARTS is addressed (or can be addressed) on a separate MIDI channel. So you simply need to set your external MIDI Controller to the MIDI channel number of the MOXF PART you want to play with that set of keys. Plug one sustain pedal into the external keyboard controller and your FC3 into the MOXF. Each Controller (key set) will control the selected/MIDI channel assigned PART.

For example, the sound in PART 1 can be played by the MOXF (Press Track [1]) and by setting your external MIDI Controller to transmit on MIDI channel 2 you can play the MOXF sound assigned to PART 2 (MIDI Channel 2).

Connect the MIDI OUT of your controller to the MIDI IN of your MOXF using a standard 5-pin MIDI cable.
Set the MOXF's MIDI IN/OUT = MIDI
Press [UTILITY]
Press [F6] MIDI
Press [SF4] OTHER
MIDI IN/OUT = MIDI