From ESRI desktop help.
The link is at the bottom and
there is more than one possible approach.
"Polygons from the geometry of other features
1.In ArcMap, click the Edit tool on the Editor toolbar.
2.Select the features whose geometry you want to use to construct new polygon features.
3.On the Editor toolbar, click the Task drop-down arrow and click Create New Feature.
4.On the Editor toolbar, click the Target drop-down arrow and click the polygon feature class in which you want to create a new feature. The selected features and the target layer cannot be from the same feature class.
5.On the Topology toolbar, click the Construct Features button .
Optionally, type a cluster tolerance.
6.Choose whether to create new polygons from the selected features, to create new polygons considering the boundaries of existing polygons as input geometry, or to split features where selected lines or polygons cross them.
The new features are created in the target feature class."