Open both files. In the file that contains the page you want to import into the other file select the page in the pages panel then use the Move Pages command from the flyout menu and choose the other file as the destination. Note that if you use master page names or style names that are the same in both files, the definitions in the destination will be used.
OR, since you framed it as "import" I'll note that you can also Place a page (or more) from another InDesign file using File > Place. This will essentially place a static copy of the page, linked to the original file, and you can use Edit Original function to open the other file for editing, if needed, just like any other linked content.