PC, no responses, eh? Let me bump it up for you, with a couple of suggestions.
First, you know how to do this? See the screen print below; that can be found on the free agents page if you are the commissioner.
Second, the only problem that I can see with the dates is that birth years may appear unrealistic; I was recently discussing this with someone else after noticing that Lou Gehrig, for example, was "born" in 1985 in order to be the 23 years of age that he was when I imported the 1927 Yankees into a game dated 2009.
Third, I am unable to answer your question about ratings adjustments, alas.
Last, here are two threads that you may be interested in:
Team Rosters from different seasons
Importing player one at a time