Custom attributes in HTML elements

Hi,
I need to add a few custom attributes in some HTML elements, which is
the best way to do it in order to keep validating with w3c validation
service?.
I want my page validates with HTML 4.01 Transitional. I read that one
way is to extend de HTML 4.0 Transitional DTD and put this in the
DOCTYPE declaration of the page (as it says in
http://htmlhelp.com/tools/validator/customdtd.html), but I would like
to know if I can do it in another way so that I can have the original
and public w3c DTD in the DOCTYPE and add my customs attributes adding
a new DTD or a namespace. Can I do that?.

Regards,
Claudia.