Steps one and two
Tap the page thumbnail in the Documents view and choose Add empty page to add a new empty page or Duplicate to add a duplicate page.
Tap the + icon at the bottom of the Documents view to add a new page to your document
This will take you to the Document picker for you to select a new page to add to your document.
This is useful when you want to add another page from a template like a presentation for example.
The slide is added to your presentation, and appears beneath the last slide in the thumbnail view.
The slide doesn't have to be from the same presentation. To add one from another presentation just tap on the document path - the breadcrumbs to take you up a level.
When you add the new slide it will automatically adopt the same theme as the new presentation...