Can I take my bicycle on the RER going from CDG Roissy to Paris (Les Halles)?
Yes you can take your bike onto RER trains. It's asked that you don't do this during morning/evening rush hours 06:30-09:00 and 16:30 - 19:00 and that you use the train cars that have bicycle spaces and for those trains that don't, use either the lead or tail cars which have extra space for bikes/luggage/wheelchairs. (See this bikes on Paris trains article for more information.)
At CDG if you're boarding from Terminal 2 train station you'll have plenty of time to check out the train cars for the RER B train, since this is the terminus/departure station. The next train to leave will be parked at the train platform usually for about 10+ minutes before it departs so you can walk up/down to find the right train car to bring your bike aboard.
There's elevator access to the train platforms at both CDG train stations and at Chatelet les Halles. Inside Chatelet les Halles you can look for the wheelchair access gates to pass through with your bike, which are usually right next to the staff windows at either end of fare gates.