Yes, you can upload a PDF to design pages in MagTitan (you do it like adding any other image to the canvas), however if you can avoid using PDF's in MagTitan you will be far better off.
While PDF's can be added very quickly, because all PDF's are is a single "picture" it takes away much of of the value the MagTitan software can deliver, making your end product, from a reading perspective, not much different than any print other replica software. This is because on smaller devices the PDF will get shrunk down to fit, causing it to be too small to be easily read. Then on larger monitors it would be stretched, causing distortion and lack of sharpness of the text.
The best practice with MagTitan is to take the elements (text, pics, video etc) that you originally used and redesign your pages inside MagTitan. This will allow the software to dynamically adjust your pages so they look beautiful and can be easily read regardless of the device used. Additionally, by having all of the elements separated individually, you can animate them to create an amazing "experience" for your readers.