Generally, NO. A schedule reports what is in the model... if you were to arbitrarily start adding rows, where would the related component reside in the model?
However, the behavior depends on the type of schedule. Room schedules, for example (perhaps the only example) allows the user to define Rooms to later be placed. Note, with rooms, you can delete the room from the model, and the schedule will report 'not placed'.
Martin Schmid
Product Line Manager
Mechanical Detailing and Electrical Design
Architecture, Engineering, and Construction
Autodesk, Inc.