This small utility does exactly what it says: converts a text usbmon capture (in either the 't' or 'u' format) to a pcap file which can then be analysed in Wireshark.
Example:
# cat /sys/kernel/debug/usbmon/1u >1u.log
^C
$ usbmon2pcap -o 1u.pcap 1u.log
Note that usbmon's text output is limited to 32 bytes of data in any packet, so you are likely to see "packet size limited during capture" in Wireshark.