I assume you want to add tremolo to the voices themselves, as opposed to adding tremolo to a song.
See page 6 of the
MOXF6/MOXF8 Reference Manual:
LFO (Low Frequency Oscillator)
As its name suggests, the LFO produces a wave of a low frequency. These waves can be used to vary the pitch, filter or amplitude of each Element to create effects such as vibrato, wah and tremolo. LFO can be set independently for each Element; it can also be set globally for all Elements. LFO related parameters can be set in the LFO display (pages 33 and 45) of Voice Common Edit or Voice Element Edit.
For a more detailed description, see section "1-2-11 LFO (Low-Frequency Oscillator)" that begins on page 34 of the
MOXF6/MOXF8 Synthesizer Parameter Manual.
If you would like to add tremolo to a song, see the section about "Song Job" that begins on page 87 of the
MOXF6/MOXF8 Reference Manual, especially section "05: Create Continuous Data" on page 92.