sibi.george,
You can use the Excel Add-in to import User Stories set up how you would like. More information about it is available, here: CA Agile Central Add-in for Excel Installation & User Guide | CA Agile Central Help
For PortfolioItems and Iterations, you would set it up like:
Alternatively you may opt in to the Work Item Templates beta, Product Announcement - Work Item Templates . While currently the template does not include Portfolio Items or Iterations, you can provide feedback directly to the Product Owners about adding it.
Hope that helps.
Michael