When I did basketball, I quickly developed a "Yes!" that I probably owe Marv Albert a royalty for, but I swear it was completely natural. (I would never suggest I was as good a basketball guy as Marv, anyhow.)
The other sports, and my public address work: just me, beginning to end. There is too much to keep track of when you're doing live sports to throw impressions into the mix. Mind you, when you're doing baseball, of *course* you hope that someone at home says "hey, he sounds like Vin Scully," because my gawd, is there a bigger compliment?
A few years after my college broadcasting career, I took to heart the words of Olbermann and Patrick in their book The Big Show when they said that trying to sound like someone else meant you were passing up the opportunity to potentially sound like someone better.