What is the rest of your water like?
Why do you need the carbon?
While I might understand it for city water with the chlorine, well water either has iron or smell... and both of those shorten the life of carbon faster than chlorine does..
Oh and forgot to add,,,
Flow rates of the two are different.
Resin has say a flow rate of 6gpm per cubic foot and a back wash rate of 3gpm while carbon has 3gpm for iron and h2s and 6gpm for chlorine per cubic foot and back wash of 4gpm.
If you are adding a few inches I my self do not think that it is going to do much in the way of removing any thing from the water or at least any thing that it should be removing.
In line house filters are more for sand and the like, not carbon.