In a performance, can i put an arp on a single voice instead of the whole performance?

Arpeggios are always assigned to a single PART of a Performance. There are four Parts, each can be assigned its own arpeggiator because there are four separate arpeggio units.

If so, will i be able to control the channel from an external midi controller.

No, not while in Performance mode. This is because your external controller can only send in MIDI note-on events that will play all four Parts of the Performance. Unlike the MOXF's keyboard (which is not using MIDI to trigger itself) the external keyboard cannot tell which Parts to trigger, which Note Limits to respect, which Parts to not trigger. To an external MIDI device all four PARTs of your Performance are on the same MIDI channel.

Here's what you can do to accomplish your goal:
_ COPY the Performance to a blank SONG or PATTERN MIXING location.
_ Set the PART you want to control from your external controller on a separate MIDI RECEIVE CHANNEL (something you can do in a MIXING, that you cannot in a Performance).

If you set all the Parts you want to play from the MOXF keyboard to channel 1 and the Part you want to control on channel 2, you can then set your external controller to channel 2.

COPY PERFORMANCE to MIXING.
Call up the blank Song or Pattern
Press [MIXING]
Press [JOB]
Press [F3] COPY
Press [SF3] PERFORM
Mark the attributes you want to copy to Parts: 1-4
Execute.

This will place all four Parts in a Mixing, each assigned to MIDI Receive Channel 1
Now you can change the MIDI Receive Channel of the Part you want to play with the external controller to ch 2.

Press [EDIT]
Press the PART 1-4
Press [F1] VOICE
Press [SF2] MODE
Set Receive Channel = 2
Press [STORE]