I think what Bhaveshn is trying to achieve is this :
Imagine a trangular stiffening web on a casting. If the web then has a draft angle applied to the side faces the top face will vary in width. What you now need to do is completely remove the flat top face by applying a varying radius fillet that is tangential to both side faces and the top face - as the pattern maker may do with his sand paper.
As far as I know this is not possible with the fillet command - unless it's been added as an option in V19 (or V20?).
It is something that is in Catia and also, I think, Solid Works.
Try using a variable radius fillet and adjusting the radii.
I just tried creating a blue surf and using the replace face command but couldn't get it to work.
Bhaveshn,
You could try building the feature as surfaces, then uniting them with your solid body.
perhaps someone should submit an enhancement request.