Wouldn't that make a confusing mess of a thread?
When you merge threads, the posts are put in date and time order, which could be confusing when reading it as to who is responding to whom.
Are the spammers hitting the existing introduction threads or are they creating new ones? If the latter, couldn't you just have new members moderated for the first few posts?
From past experience, users on the sites I've been part of quite like their own intro threads as they get 'individual attention' from members asking them questions and they can then revisit them in years to come to celebrate their Xth anniversary since they joined.