I had a feeling that was the case - that the import/export feature is not available in calibre portable - ideally it shouldn't even have the import/export item in the drop/down, but I see it is.
- Close all instances of calibre.
- Copy your existing libraries into the Calibre Portable folder, and copy the contents of \Users\your user name\AppData\Roaming\calibre into Calibre Portable\Calibre Settings.
- Start calibre-portable and use Library->Switch create to hook the libraries you just copied.
BR