Yes, you absolutely can. You'll get status miles and regular miles.

Not sure where you are booking, but if on the United site, there is a place to add your Aeroplan number, you'll just have to use the pulldown menu to select Aeroplan as the program you want to credit to, and then you should be good. Orbitz even allows you to do this. If booking with a TA, you can just give them your number, and they should be able to handle it.

If you can't do this at booking, or your ticket was already booked, just call United Reservations, give them the record locator, and they should be able to add this for you. Also, I beleive you can look up your itinerary on United's Website with your record locator, and you should be able to add your aeroplan number there.