The date filter can handle TAI64N
mutate { add_field => { "someField" => "400000005cc363f11d3becc4" } }
date { match => [ "someField", "TAI64N" ] }
will get you
"@timestamp" => 2019-04-26T20:02:47.490Z,
If you want to format @timestamp into another field with a particular format you can search the forum for examples using strftime in a ruby filter.