Yes/No/Maybe.
Yes if the OPC Server can directly read a 'virtual' Ethernet port, and the target device/serial-to-Ethernet converter formats packets properly.
No if the above is not true.
Maybe if the OPC Server can also read data from 'Virtual' serial ports, and you pair the 'serial-to-Ethernet' converter on various ends to an 'Ethernet-to-virtual-serial' coverter on the OPC Server end.
Detail: Any number of manufacturers can make a module that simply encapsulates a serial (or CAN, or ProfiBus, or DeviceNet, or AnyOtherNetYouCanThinkOf) into an Ethernet TCP(or UDP)/IP Packet, but on the receiving end, most of the time the encapsulation must be stripped out somehow, unless the target (your OPC Server in this case) can understand a particular encapsulation format. If the target can't understand the encapsulation, then a fall-back is to use a sister device on the target that can strip away the TCP(or UDP)/IP info, and virtualize the communications back down to an expected format.
Example: To send (and receive) generic "XYZ" serial data over Ethernet from a transmitter to a receiver, some converter will connect to the serial COMx: port of the transmitter, wrap the required packet headers around it, and pump it out over Ethernet to a target. The target can either understand how to strip away the header information, and deal with it (unlikely), or will be running some software (usually provided by the converter's manufacturer) to strip away the encapsulation information, and present the actual data to the target as a virtual COMx: port. You might then read the information natively by the OPC Server by looking for a normal, wire-connected serial port on, say, COM9: for example.
