Check the fare conditions on Matrix (ITA Software) and check the fare rules. Some fares allow such a combination, others don't. If the rules allow any TA can book this for you.
These are he fare rules:
Matrix - ITA Software by Google
Look at Category 10
"FARES MAY BE COMBINED ON A HALF ROUND TRIP BASIS "
"WHEN FARES OF MIXED BOOKING CLASSES ARE COMBINED ON A HALF RETURN BASIS THE FARE CONDITIONS OF THE MOST RESTRICTIVE FARE WILL APPLY"