Well, to beging with you're wasting your time adding that particular DTD to FrontPage 2000 pages since the page code will be anything but XHTML compliant.
There are ways of migrating the XHTML via FrontPage 2000 but they are a bit involved and not terribly efficient.
To begin with FrontPage 2000 generates some quite odd code. There are ways of 'cleaning' that code which strips out the proprietary tags but this can sometimes break your pages. Ususally it doesn't, but it's only fair to warn you that it can.
Now, rather than me typing and waffling on, try looking here to begin with for information on writing accessible code with FrontPage 2000 (including modifying the DTD in your page template).
There is an interesting article at this link click here that contains instructions on how to integrate HTML Tidy into FrontPage. HTML Tidy is a small program that does what its name suggests.
The above should get you up and running, but if you have any questions feel free to ask.