Consolidate old posts from blog into new

I don’t know if there’s even a way to do this…but thought I’d ask…

I have recently exported all my old Blogger posts and imported them into my WordPress blog. I’m thinking of closing down my Blogger blog. I want to condense things. The thing is they are old and in a bit of a different voice than my current blog…which I’m fine with, especially since they are older posts, for the most part…but I feel like it would make more sense if people are digging around in old posts, if there were a bit of text at the beginning of each of them that gave people a heads up that they originally came from another blog.

I do have them all in the category “Blogger”, but not everyone notices the category names, nor would they necessarily understand what that meant. So a sentence or 2 intro would give a nice explanation, I think.

But there are 500 or so of them and I REALLY don’t want to have to do all that manually!

So my question is…is there an easy way to insert a bit of text into JUST one specific category of posts?