Hello farrei.
No, Siemens S7-400 cannot be used with the jx-server.
But you can probably let the Adventrol software connect to multiple OPC servers. You should check up on this, but I would be surprised if it is not the case.
So the solution is to install an additional OPC Server for connecting to the S7 PLC.
Ethernet is the obvious solution since there is an ethernet CP in the S7-400 rack.
You need to get Simatic Net IE LEAN which contains an OPC Server.
You should check the exact type no of the ethernet CP.
The use of multiple OPC servers for one or several OPC Clients is exactly the strength of OPC.