what do you mean by collapse or extend? an include file is just that, it includes the contents of another file. If you mean you want the option of whether or not to include the file at all, then you would do something like:
<% if someCondition = True then %> <!--#include file="somefile.asp"--> <% end if %>
note that even doing this, the server must parse the code in the include whether or not it's contents are actually displayed.
Just the same way as default.asp shows the categories with the minus/plus icon, so people can choose which include file they want to have shown - not sure it´s possible...!?
You can't programmatically decide what files to include. Included files are added in the code base for the page before any asp code is executed.
You can selectively execute included code using something like <% if somecondition then %> <!--#include file="somefile.asp"--> <% end if %>
But the code from all files used this way are included, and if you have functions or subs, or declared variables of the same name in different include files your code will fail.