If content is not included in the channel, there is usually a very good reason. Video channels are often designed to access the full episodes available on a site, so sometimes the designers may choose not include clips and extras, if it will be more work to maintain and/or require extra code that could make the channel less stable.
Certain content may also be purposefully blocked within the code of the plugin by the channel designer. Usually the reason certain content is not available is because that content may not be compatible with Plex and would cause errors or just not work. There is no point in providing access to content in the channel that will not work.