How can I add soft credits to a Recurring Contribution Series
I don't think this is currently possible, but I've recently been up to my elbows in soft credit customizations.

I think the answer is an extension that checks whether the first gift is soft credited.  If so, it should apply an identical soft credit to succeeding gifts.

I recently released the extension Soft credit Custom Fields.  You could use soft credit custom fields to further specify other options.  E.g. only soft credit the first X contributions; if the contribution amount is increased later, does the soft credit increase as well; etc.