man ettercap:
-F, --filter <FILE>
Load the filter from the file <FILE>. The filter must be compiled with etterfilter(8). The utility will compile the filter script and produce an ettercap-compliant binary filter file. Read the etterfilter(8) man page for the list of functions you can use inside a filter script.
NOTE: these filters are different from those set with --pcapfilter. An ettercap filter is a content filter and can modify the payload of a packet before forwarding it. Pcap filter are used to capture only certain packets.
NOTE: you can use filters on pcapfile to modify them and save to another file, but in this case you have to pay attention on what you are doing, since ettercap will not recalculate checksums, nor split packets exceeding the mtu (snaplen) nor anything like that.
Das sollte es doch sein, oder?
LG