Modbus TCP Protocol

Greetings of the day,
As of Modbus protocol, it can access
1: Output coil status
2: Input coil status
3: Internal Registers
4: Holding registers

As the memory addressing in AC500 is like %MW0.0 if I add some values to that register while using Modbus protocol which I should use Holding Register or Internal registers

Also, I'm now using PM554 ETH CPU as it can store approx 600 Memory Double Words can I increase that by inserting a memory? IF it is possible how I will address the new memories?

If anyone can help me in these troubles, please

Regards

Amal