Can I connect wall sconces to an existing switched circuit?
Technically, yes you can tie into those outlets for power. 

Whether or not it is safe (and not against code) to do so would depend on what other loads are already on the circuit (will adding the lights overload the circuit?) and the size of the electrical boxes where the outlets are located (are they large enough to accommodate the wires from another branch?).

You would need to do some load calculations taking into consideration the wire gauge as well as box fill calculations.  This information is available in the NEC - always a handy reference to have available when doing this type of work.

If in doubt, it's always safest to consult a licensed electrician.