Most scanners come with OCR software (consult the scanner's manual for the details). This type of software converts the scanned image to electronic text, which you can open in a word processing program like Microsoft Word.
In Word, save the file out of that program as a text-only file.
Next, open Final Draft. Go to File > Open (on Windows, under the Files of Type dropdown menu, select Text Documents). Select the text file and click Open. You will be prompted to convert to script or text. Choose script. This will import the scanned text into a new script document.