Hi,
if there are standard ASPX pages, you can add them to the system as ASPX templates (http://www.kentico.com/docs/devguide/creating_a_new_aspx_page_templ.htm you can skip the code development steps). the other option is to re-write the layout manually to the Kentico CMS page templates. Unfortunately it is not able to create a general procedure for converting or importing existing aspx sites to Kentico - every developer has his own style of coding.

Best Regards,
Juraj Ondrus
Support Engineer