Is it possible with ArcGIS Pro SDK to add content of mxd and pitemx files to map? For example, for mxd as a group layer containing all layers inside mxd.
ItemFactory and Import
Contents of an mxd can be imported into Pro. This topic above can help you with it. Using "IProjectMultiItem" interface, you will be able to import the map and layout from an mxd into Pro.
Regarding pitemx files, if the online/portal item that is referenced by the .pitemx file is something that can be added to Pro, then yes, it can be imported into Pro also.
Retrieving data ...