wordpress create a new file which can get data from wordpress and use functions

[Synperuna]

I'm making the export xml file from admin wordpress. In a custom admin page I built , it have a button to export xml file. The export button will go to a url of export file , which will get data and display as xml file.The problem is that wordpress does not allow to create a new file which can get data and use function of wordpress. The export function is the part of a custom wordpress theme, so the export file must be predefined! I can create a wordpress page template to export data. but it need to create a new page and use the export page template.I can create a wordpress admin page , but it look like alway to load admin screen, so it can't be turn into a xml file.The content of export xml file is almost data from wordpress options and a custom database which theme built!Anyone have idea to solve this problem? Thanks!