I have a set of concrete stairs, 4 steps, that I want to widen. My question is does anything have to be done to where new concrete meets existing concrete, or should I just drill into it and extend rebar into new form ?
Also, I intend on topping new and old stairs with pavers. The old stairs are not quite level and true, can I built them up level with new stairs with the "thinset"? That I intend to use to set the pavers ?

Normally this is not done this way. How much in width are you adding?

If the old stairs are not quite level and true and this is a result of the steps leaning back into the house then need to re do the whole step system.

Once they start to lean back its pretty much a goner in time.

So many steps start to lean back in time because of the backfill against the house wall and or no rebar was used to pin the back of the steps.

The other reason this happens a lot is because the ground thaws quicker against the house and the front it still frozen and heaves pushing up in the front and sinking in the back little by little over the years.

If your scenario is above then redo the whole step.