Is it possible to modify an open order w/o adding a new one?

 
If I buy, say, 1000 units and then sell 100 and 200 units, is it possible to have my original position open with 700 units instead of having 3 open positions, one long for 1000 units and two short for 100 and 200 units?
 
tfoller:
If I buy, say, 1000 units and then sell 100 and 200 units, is it possible to have my original position open with 700 units instead of having 3 open positions, one long for 1000 units and two short for 100 and 200 units?
You can partially close an order, stipulating the volume to close.
Reason: