You can send an insteon command to a button referencing the added device.
Take a look at the thread below for how to add the separate buttons as devices. Let me know if you still have problems.
From the thread, the steps are
1) Go to add a new device
2) Select a new group
3) Enter a name
4) Hit save
5) When prompted to add a new record, say yes
6) The EZSrve will go into linking mode, press the corresponding button on the KPL you want linked for about 6 seconds
7) The EZSrve will say it found the device and stop linking.