unable to specify a custom dtd

[macbeth]

hi,
according to inline help about the public DocTypeMode DocType property:

" you can supply a string for the formal public identifier (FPI). / For example: doctype: "-//ACME//DTD HTML 3.14159//EN" "

How i can specify my custom dtd in the wrapper ? It is possible ?

I want be able to validate a xhtml document extended to support schema.org (check for more info last post of : http://stackoverflow.com/questions/3659778/can-i-add-microdata-from-html5-to-a-xhtml-strict-site-and-still-be-compliant )

Thanks

Tommaso