No, you can't merge accounts. You can manually export data to Zotero RDF with files and import into another account, but that will, at the least, break links to citations in existing word processor documents and reset Date Added/Modified values.
Generally speaking, you should never create multiple accounts. You can always change your username and add an email address to an existing account.
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Generally speaking, you should never create multiple accounts. You can always change your username and add an email address to an existing account.