There are no carriage returns in Word. A word document consists of
character, words, paragraphs, and sections. If you are pounding the "enter"
key at the end of each line then you are basically using a powerful and
quite capable word processor as a typewriter. See:
http://www.shaunakelly.com/word/concepts/starttyping/index.html
for a few basic tips on using Word.
Back to your question. Select the text the you want to justifiy. With the
text selected, press Edit>Replace. Put ^p in the find what field, put a
space in the replace with field and click replace all. When Word reports
the number of replacements and asks if you want to search the rest of the
document, click no.
Now select the text again and press CTRL+j
