Que tal
awk -F"\t" '
NR == 1 {$2 = "datetime" OFS "monthyear" OFS "time"
}
NR > 1 {split ($2, T, "[/ ]")
$2 = $2 OFS T[1] "/" T[3] OFS T[4]
}
1
' OFS="\t" file
id datetime monthyear time userId itemId
1 12/1/17 8:32 12/17 8:32 2232 tv
2 1/12/18 10:18 1/18 10:18 3232 fdfs
3 2/9/18 10:50 2/18 10:50 232 fdsf
4 3/6/18 12:35 3/18 12:35 345456 fdg