As you're working in Lucidchart, you may need to use content from other Lucidchart documents you've previously created. Rather than copying and pasting manually, you can quickly import pages from other Lucidchart documents.
To do this, select "Insert" from the menu bar and then select "Import page." You'll be able to search for your document by name or browse through your folder tree, including documents that have been shared with you.
Go ahead and browse through and select the document you want to attach. Once you've selected this particular page, you'll be prompted with the thumbnail to ensure you're importing the right content. Then, click "Select."