Add a Profibus port to the Beckhoff PLC. It is quite cheap by the way.
There is no Profinet module with Beckhof, so you cannot go that way.
The ethernet ports on S7 and Beckhoff are not so easy to get to talk to each other, but there are some possibilities (Modbus TCP for example). But I dont recommend you to go that way. It will be more expensive, less supported, and I dont see any obvious advantages compared against Profibus.