In general (unless docx4j were to implement Word's undocumented page layout algorithm), there is no way to no where the second "page" starts.
If however. your document contained a manual page break XML element, you could locate this, and insert your content after it. Similarly, you knew what content was already on the "second" page.
Its prob worth mentioning here that there are 2 ways to find a specific bit of content. One is iterating through/ traversing the document content. The other is via XPath. See the Getting Started guide on this.