[Zigûr]

We know how mendacious and manipulative Saruman was, so it seems possible in my opinion that the early "encroachments" upon Rohan and supporting of Rohan's enemies, which would presumably constitute the Dunlendings for the most part, were either done in an affectedly non-aggressive manner or that Saruman, despite claiming Isengard, did not claim responsibility for them. Perhaps they were done in secret. Appendix A is meant to have been written in the Fourth Age, is it not? So these actions might not have been recognised for what they were at the time. Certainly Saruman's treachery does not appear to have been common knowledge until after Gandalf escaped from Orthanc.

On the other hand perhaps Saruman presented these actions as simply being defensive measures which were, he may have claimed, in the best interests of the local area in general, "protecting his territory" (seizing land beyond his own borders) and "strengthening friendships/alliances/etc in the region" (giving materiel to the Dunlendings). He could easily have been telling the Rohirrim one thing and the Dunlendings another.

Thus I think it's possible that Saruman could be attacking Rohan and trying to forge an "alliance" with them simultaneously, if he was sufficiently crafty. Thus it could occur in Théoden's reign despite the fact that he'd already actually begun troubling Rohan by that point. Politics can be a very messy business, and Saruman is one of the most "political" characters in all of Professor Tolkien's work.