Install ethereal and go to the same directory where it's installed. There's a command line tool called mergecap and you can convert the libpcap format to either netmon1, netmon2, or any other format you wish under the following choices:
libpcap - libpcap (tcpdump, Ethereal, etc.)
rh6_1libpcap - RedHat Linux 6.1 libpcap (tcpdump)
suse6_3libpcap - SuSE Linux 6.3 libpcap (tcpdump)
modlibpcap - modified libpcap (tcpdump)
nokialibpcap - Nokia libpcap (tcpdump)
lanalyzer - Novell LANalyzer
ngsniffer - Network Associates Sniffer (DOS-based)
snoop - Sun snoop
netmon1 - Microsoft Network Monitor 1.x
netmon2 - Microsoft Network Monitor 2.x
ngwsniffer_1_1 - Network Associates Sniffer (Windows-based) 1.1
ngwsniffer_2_0 - Network Associates Sniffer (Windows-based) 2.00x
nettl - HP-UX nettl trace
visual - Visual Networks traffic capture
5views - Accellent 5Views capture
niobserverv9 - Network Instruments Observer version 9
rf5 - Tektronix K12xx 32-bit .rf5 format