Question: title says it all. Internet is full of examples how to convert `FILETIME` to `TDateTime` but not the other way around.

Answer: 
1. Call `DateTimeToSystemTime` from `System.SysUtils`.
2. Call `SystemTimeToFileTime` from `Winapi.Windows`.

If your original date time value is in local time, then you will need to convert it to UTC before calling `DateTimeToSystemTime`.
You can look at implementation of `ConvertDateTimeToFileTime` in `System.IOUtils`.
It is a private member of `TDirectory` record.
But you can use it as a reference in order to implement all you want in one function.