A family of Microsoft word processing software products for creating web, email, and print documents.
The problem you are having is that Word does not work with "lines" but with paragraphs. If you pressed the Enter key at the end of each line, then you do have a paragraph for each line.
If that is the case, try the following:
Press Cmd+H for Replace.
For find type: ^p
For replace: type ^p^p
That will insert a blank paragraph after each existing one. You'll have to press the Enter key once, yourself, at the beginning of the document to insert the blank paragraph there.
If you did not press Enter at the end of each line originally write back and someone can come up with a macro.