Desing issues for inserting foreign pages

Hi, I was just about to start a new project which required to insert some new functionalities (namely a Shopping Cart and other stuff) to an already existing web page.
My question is how can I do this if I have no access to the original web page? In this scenario, the page will simply have to point to another web page that will have all the functionalities required so that the web site webmaster has to modify as little as possible.
So, is there a way of doing this?? How can I ease to use of diffrent languages (ASP.NET, PHP, etc). I was thinking in the way of "a la" portlet, but does that mean that I have to build custom controls for each specific lenguage so that each webmaster can simply drop a control or something into is page and the rest will be taken care of.

All thought and comments are welcome - and needed :-)