You need to break it down to points if they're good points and reconstruct. Polylines are made from paths, polygons are made from rings. Although they are created in a similar way they are not compatable, hence your error.

Go through each point on the line adding a point to your output array and then insert.

here's a post that might help http://gis.stackexchange.com/questions/26369/get-all-the-points-of-a-polyline

This should work:

    feat = row.getValue(shapefieldname) # input line
    PArray = arcpy.Array() # new polygon
    partnum = 0
    for part in feat:
    	for pnt in feat.getPart(partnum):
    		PArray.add(arcpy.Point(pnt.X,pnt.Y))
        # you will need to check that the first and last point are the same
    	partnum += 1
    OutPoly = arcpy.Polygon(PArray) # now it's a polygon