Use some external envelopes on the buss mod input.
Or use a simple CV pedal to introduce the LFO modulation.
Or use the Osc 3 as LFO an use the LFO with a slow triangle wave with the sync on KB (keyboard) as delaying modulation source. But this would cause problems, when the note is played longer then 1/4 of the (real) LFO wave cycle.