Custom extension for specific tags

[GunterO]

Hello,
Is it possible to add an extension similar to Syndication.cs myself? And if yes, how?
I need one for elements that start with "torznab:"
e.g.

<torznab:attr name="category" value="2010" />

And also one for non-default sub-elements of <item> like :
e.g.

<item>
      <size>4294967296</size>

Thanks!