Does using an existing attic exhaust pipe to also serve air flow from a range hood cause smells/micro particles of oil/soot/etc to get into attic?
Yes, this will cause a problem. 

You say, “with an opening in the side of this pipe...” If there isn’t a solid pipe (without openings) in your new exhaust pipe, it will partially vent kitchen air into your attic. (Good practice calls for the pipe joints to be lapped in the direction of the exhaust and sealed...no openings.)

Having a “gravity” vent in your attic does not allow for backdraft flaps...so it can’t be fixed. 

These are two separate venting procedures. One is power vented and one is gravity. They cannot be combined. The power exhaust vent will blow smells into your attic. 

I’d check with your Building Department and they’ll tell you that the new kitchen exhaust vent must be vented to the exterior...without “openings in the side of the pipe”.