It's not wrong to add posts to an existing thread, but creating an additional thread on a topic that already had one or more active threads is considered by many to be disrespectful of other FT'ers, and is against the Forum's Guidelines & Rules:

Use the Search Feature - link to this guideline
Before starting a thread, use the search feature to see if there's a recent thread on your topic. Many questions come up repeatedly on FlyerTalk. If you're wondering about a specific Hilton property in Chicago, for example, there's a good chance a thread - or 10 - already exists. The search link is located in the navigation menu near the top of the Forums page. Keeping information on the same topic confined to as few threads as possible helps members quickly find the information they need.