problem parsing html as php with .htaccess

[HarryPW]

I'm using a CMS witch save all the files as a .html document
Normally I put a .htaccess file in the root with:

AddType application/x-httpd-php .php .htm .html
or
AddHandler php-fastcgi .html

and use my own error404 document from the CMS with:
ErrorDocument 404 /error404.html

but this all doesn't work under ispCP.

Anyone a suggestion or a solution

Thanx
Harry