Expanding on an earlier given answer is fine and IMHO should be encouraged. People can always look at the edit history so they can see what has changed. Also when you edit an answer, any upvotes on it (which are normally locked after voting) can be undone by the voter in case he/she now disagree with your answer.
Some people prefer to explicitly mention their edits in their answer text, for example by adding the text "EDIT" at the end of the original text and then add new text (see for example this question or this answer), but this isn't mandatory. Personally I only do this if the edit is substantial (but still related to the original answer).