Sure. Insert a seat post that will fit as is, so that about an inch or so is in the seat tube.Then gently roll the post back against the slot, and move side to side. That should open it up enough to get the slot edges straight and allow a 26.6 post to fit.

I recently did this for a Mercian that came with a 26.8 post that felt snug, but I knew the seat tube was standard Reynolds, so it took a 27.2 post. I was able to get the seat tube opened enough to use the right post.