Thank you, Prof. Adrian for answering my question.

Yes, Wireshark or tcpdump can be used to export the traffic to a pcap format.
However, those tools are not suitable in our case since they can't save the traffic based on a flow like in NFStream.
In NFStream, the traffic will be grouped to each flow, then everytime a new packet comes, NFStream will check whether this packet belongs to the existing flow or a new flow.

In our case, we have to group all live traffic to each flow, then if the flow is finish, we need to do two things, firstly, we will extract it to pandas and analyze the traffic, then secondly, we also have to extract the finished flow to a pcap file. NFStream is very suitable for us in this case, however, we don't know how to modify the NFStream to extract the finished flow not only to the pandas, but also to a pcap.