how to export a stripped capture file to k12 ?

edit

Or is there any other solution to my problem?

Export the file to a format that supports the notion of a packet having an "actual length" and a "captured length".

k12 text files are NOT such a format. That is the cause of your problem; it is not something that the Wireshark developer's can fix (we don't define that format, Tektronix did).

One such format is called "pcap format". Another such format is called "pcapng format". Wireshark is capable of reading and writing both those formats.