Can Stormtroopers shoot like this.

1. Aim
2. Roll dice
3. Reroll 3 dice (Aim + Precise 1)
4. turn surges to hits
5. Then if they have impact (e.g HH-12 has impact 3) turn some hits to crits.

Is that correct ?

Yes. But Dodge and Cover are applied between the Reroll and Modify dice step.

RRG pg 14 Attack steps.

Converting surges to hits happens after re-rolls and before Dodge and Cover are removed. Then after Dodge and Cover removal you apply modifiers. I have seen too many people get this little bit wrong.

Technically, the surges turn to hits immediately after steps 2 and 3 -- they're part of the rolling process, not the modify dice step.