You cannot add field points using manual configuration. Only the auto-discovery procedure is supported. In order to add a Modbus device integration to a network engine vendor integration, you must have the VMD file that represents the full database for your specific Modbus device.
For each Modbus integration on the network engine, you can have one or more VMD files that include a specific database related to one or more Modbus devices. The only restriction is that all the devices from different manufacturers must use the same communication rate (baud rate, stop bits, data bits, and parity). For example, if on a single Modbus vendor trunk, you have two Caterpillar generators, four Square D meters, and a Fireye® boiler, add different VMD files to your Modbus Integration object as resources. This example gives you seven devices defined under the Modbus Integration object.