There has been plenty of efforts to do this but for one reason or other, all have foundered or not been carried through.
On the subject of Highlander, what came first, Highlander or Casca? Highlander was written originally by a Canadian university student in around 1983 or 1984. Casca had been written and published in 1979. Who’s to say this student hadn’t read Casca and borrowed the general idea of an immortal?
Besides, Casca as a series would be ill-served by the ‘flashback’ method. It’s already been done many times over. What Casca’s strength is, is the immersive there-at-the-moment series, of a warrior at that period of time fighting at that time. To reduce it to the easy-to-do and cheaper option of setting it in present day and ‘flashback’ is a cop-out, as far as I, and many others, are concerned.