Amtrak also transfers 5000:10000 to HHonors, and because CO transfers 5000:5000 to Amtrak it is possible to do a two step conversion with no fees...

CO:AGR:HH 5,000:5,000:10,000

However, the second transfer is very slow, and limited to 25,000 points per calendar year (50,000 for Amtrak elite).

Over time programs that function merely as conduits for transfers between other programs find ways to eliminate or restrict such transactions, because it doesn't generate any business for the sponsor.