Integration into existing site???

Hi chaps,

If I have a great little site already developed and then need/wish to add a facility to sell stuff, can I use Zen Cart? Do I have to use templates and create a whole new site, or is there code to simply embed into an existing site?

Example:
I have a little site saltmoorhouse.co.uk. Currently, it is just a php built site doing the usual - header/body/footer, using a common header and footer file per page and then putting the code relevant to each page in between.

They don't actually want to, but IF they did, can I simply go to the code I already have and stick some sort of php function calls to place the products where ever I want?

Here's hoping :)

Some example code would be greatly appreciated.

Thanks in advance.