how to set attributes on a polyline

here is what I am trying to achieve:
We have a polygon area that will delay a ship moving through that area for a period of time. I need to apply this to our hydrography map.

How we are doing this:
Each cell in the cost array equal (n sec/meter) * 1000 depending on the speed of the ship
The delay needs to be applied with the value 1 in the middle cells and the delay time / 2 in the outer rim of the polygon.

We use IExtractionOp2.Polygon to get the part of the hydrography raster that the polygon overlays
We set the entire polygon area to the value 1 using ReclassByRemap
Next I get the polygon.Boundary to get the polyline that encompasses the outer ring of the polygon.
Finally I intended to call PolygonToRaster to get a raster with the outer rim set to delay time /2

The call to PolylineToRaster fails and I think the reason is I'm passing an integer field. The documetation, I found, states it has to be "any attribute field in the Polygon". So how do I set a specific attirbute field with the values I need? The polyline doesn't even expose anything like AddAttribute!!!

Thanks