New Variable Configuation in GE Mark VIe using PSCA Serial Comm Module

In our plant, WTP is not under automation, we want to add some WTP related variables in GE Mark VIe like automatic motor control depending upon the level of the tank which is digital signal. In this running plant cable laying is tough. We want to add variables in ToolboxST using Serial Comm module(Modbus protocol). I want to know the exact procedure of variable configuration in PSCA module and communicate with the HMI also.