Two ways to do it ...
1. highlight the text in the locked thread, right click your mouse, "copy", go to new thread, click on "reply", click on the quote icon above (see the two rows of buttons above - the first row starts with B for bold, second row second from the end is the quote icon) and then paste the text between the two quote statements - that will make it stand out as a quote. Then add in your text after the second quote statement.
2. find the locked thread, find the post you want to quote and check the reply number (top left corner), then copy the URL address. Go to the new thread, click on "reply", click on the insert hyperlink icon (second button row, third from the left) and paste the URL between the url statements and then after the second url statement, type in your text and tell folks which reply in the locked thread they should go look at.
Good Luck!