Name somebody who has been impeached over something he did before he was in office.
And, btw, if Clinton had gone to trial and been found guilty in the Paula Jones case, he would NOT have been impeached for that.
That does not mean that someone may not be impeached for high crimes and misdemeanors committed before their current public office.
Impeachment is a serious punishment and only used in absolutely necessary situations.
Even One Particular seems to concur.