Is there a way to convert all comments into javadoc comments? [Eclipse/Java]

Javadocs are a specific comment format used to generate external documentation for your code. You shouldn’t be converting regular comments to Javadoc.

For example:

/**
 * Do foo with a.
 * @param a the a value
 * @return some different value
 * @see #bar
 */
public String foo(String a) {
    // blah blah blah
}

See this technical doc for more details.

If you do want to convert the comments, though, you would have to do it manually like you said. There isn’t a specific tool for this.