Add custom page in backend not by plugin

[arek1]

OK, thank you for clarification. So please back for a moment to plugins.
Because my plugin should alter all content (put special form and other elements in page content) I should use core filter:

Code:

add_filter('content','my_function');

If it is correct go to next problem, how I can restrict this plugin to alter content only one particular page?
By checking slug name or something different?

Best wishes
ArekS