I have concrete pad on east side of the house exterior. It has settled and currently slopes toward the house. It doesn't make sense to me to rip out concrete put down crusher run and then pour new concrete. Seems that existing concrete should provide good enough foundation. I could hammerdrill holes and put rebar in holes to join old pad with overpour. Need some advice.

I am assuming that this is the landing for deck steps or something similar. The present pad is settling because soil was not compacted by builder if house is 10 years or less old. Or because water from gutter down spouts is not being carried away from house.
I'd just pour new pad on top of old if minium thickness is greater than 2". Less than 2" break up old and pour new pad 4" thick. No rebar, not wire, you aren't pouring an airport runway.