This is how my playing group plays it and it's worked out very, very well.
Basically, you can combine sites from one "block" (F, T, or K) and new sites.
We errata
Ettenmoors* and
Emyn Muil* to be worded like
Ettenmoors and
Emyn Muil.
Here the details for how to build an adventure deck like this:
"An adventure deck is made up of 9 total sites from sets 11-18 and one block (i.e. sets 1-3, 4-6, or 7-10). You may not have multiple sites with the same site name, nor multiple sites of the same site number, nor more than 3 sites from sets 11-18 with the same shadow number. When playing a site on the adventure path, if you have a site from sets 1-10 with the matching site number in your adventure deck, you must play that site; otherwise you may play any site from sets 11-18 in your adventure deck. If your site from sets 1-10 is on the adventure path, you may not replace it with another site from your adventure deck. For sites from sets 1-10, since the "region cost" is included within the "shadow number", the "shadow number" cannot become less than 3 for region 2 sites nor less than 6 for region 3 sites."