How to save dreamwaver version 6 into 5.5?
You save the files in the normal way (I am assuming you have defined the site for your project) and then in DW CS 5.5, you define the site again but this time pointing to the folder where your DW CS6 files reside.
The files are all ordinary standard text files so it make no difference what version you use as long as you follow the standard procedure of defining the site in DW.
As pointed out, the files are text. If you use features that only exist in CS6, like Fluid Grid, the code will still be editable but the GUI interface will not work so you have to know what you are editing in the code.