Unfortunately, you don't. The AF100 bus is not supported by AC500.
You could possibly convert the AF100 Fieldbus to Profibus by changing the modems to CI840 and replacing the twisted pair cable with purple hose profibus cable. ( The AF100 and Profibus cable types are similar, but it is not possible to say that the AF100 cable in your plant will actually run profibus communications. )
However, the cost of doing this would be far larger than the cost of an AC500 Eco PLC. That seems a very small PLC to be connecting to what is typically quite a large number of IO signals on an AF100 bus.