The HPD-20 has a Trigger Input on the back panel (TRIG IN) for connecting up to two external pads. You can trigger any of the HPD-20's internal sounds with external pads if desired. Here's a list of compatible Roland pads/triggers:
KD-7
KD-8
KD-9
KD-80
KD-85
KD-120
KD-140
KD-180
KD-180L
KD-200
KD-220
KDA-22
KT-9
KT-10
PD-8
PD-80 /R
PD-85
PD-100
PD-105
PD-108
PD-120
PD-125
PD-128
PDA-100L
PDA-120L
PDA-100
PDA-120
PDA-140F
PDX-6
PDX-8
PDX-100
PDX-12
CY-5
CY-8
CY-12C
CY-12R/C *
CY-13R *
CY-14C
CY-14C-T
CY-15R *
CY-16R-T*
BT-1
RT-10K
RT-10S
RT-10T
* Three-zone triggering is not possible. The pad will produce dual-zone (Bow/Edge) triggering only.