If you have an immediate need, I think your best bet for a response is to contact SAS Tech Support.
The problem I'm having with your request is your insistence on using Hash tables. Even if you have a large file, a hash table might not be the best way to get a count and a sum, even with the need to do the join. On the other hand, a hash table might be the right approach. Only Tech Support can tell you for sure. Like Doc, I would use Proc Means or Proc Tabulate, both of which are highly efficient at summarizing and/or counting.
However, since you really seem to want a hash table approach, then Tech Support would be the right people to help you. They can find out ALL the specifics of your big tables and help you with a solution in a timely manner.