You could extract it with an eval command like this:
< your search > | rex field=bytes_transferred "(?<trans_amount>[\d\.]+) (?<trans_unit>\w+)" | eval kb_transferred=case(trans_unit=="KB",trans_amount, trans_unit=="MB",trans_amount*1024, trans_unit=="GB",trans_amount*1048576, trans_unit=="TB",trans_amount*1073741824)
Explained:
Split the bytes_transferred field into a field containing the amount and a field containing the unit:
| rex field=bytes_transferred "(?<trans_amount>[\d\.]+) (?<trans_unit>\w+)"
Evaluate the amount in KB based on the unit:
| eval kb_transferred=case(trans_unit=="KB",trans_amount, trans_unit=="MB",trans_amount*1024, trans_unit=="GB",trans_amount*1048576, trans_unit=="TB",trans_amount*1073741824)
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