I'm assuming you have the XP60 or XP80.
I've never actually done this, but want to figure it out, too. So I've gone to the manual (p. 57 in my XP60 manual).
In the manual under FUNCTIONS OF PATCH PARAMETERS, check out "Modulating Sounds--Adding Vibrato, Tremolo, etc.//Using controllers to change how sounds are played (LFO&Ctl)."
The manual explains how to sync your LFO rate with the sequencer's clock in both patch mode and performance mode:
"Rate (LFO rate):
and
"Ext Sync (LFO external sync)"
You'll probably also want to set "Key Sync (LFO key sync)" to "ON" so that your pulses start when you press the key.
For the "Ext Sync" options, the manual lists them: Syncing to CLK syncs the LFO delay to (1) the Patch tempo or (2) the Perform tempo or to (3) the sequencer's tempo clock, which is apparently what you want, in which case:
"set the Clock Source parameter (PERFORM/Common/Common) to SEQUENCER."
The RATE (LFO rate) will let you stipulate a NOTE VALUE in multiples of a quarter note.
That should do it. I'm anxious to try this myself.