Can I write MELSEC data on the DX2000 via Modbus/TCP communications?

Yes. However, the DX2000 requires the MATH function option (suffix code: /M1).
The procedure is as follows.

1. Enable the DX2000's Modbus server function.
2. Register communication input channels (C01 or C02) in a formula and start the MATH function.
3. Next, assign MATH channels to groups, and set them up to display screens.
4. On the MELSEC side, write to the DX's hold registers (communication input channels). The register addresses for the DX's communication input channels are 400001 and higher.

For information on registers, see the DX2000's "Communication Interface User's Manual. " It includes a list of registers. For information on MELSEC operation, see the MELSEC operation manual.
DX2000 user's manual