you could try to use editcap (in the Ethereal directory), which, according to its description, should do what you need:
"Editcap is a program that reads some or all of the captured packets from the infile, optionally converts them in various ways and writes the resulting packets to the capture outfile (or outfiles).
By default, it reads all packets from the infile and writes them to the outfile in libpcap file format.
A list of packet numbers can be specified on the command line; ranges of packet numbers can be specified as start-end, referring to all packets from start to end. The selected packets with those numbers will not be written to the capture file. If the -r flag is specified, the whole packet selection is reversed; in that case only the selected packets will be written to the capture file."