Pellets are actually very nutritious food if you use good pellets (not those a packet for a dollar one). Many peorts they could digest and expel the rest.
On the other hand carnivorous fish has the instinct to strike at only food that moves. Something like a pellet does not register as food at all.
The main trick to let a predator fish eat pellets is to let it recognise it is food. You can either let it see you eat pellets or you can have tankmates for the demonstration. Just note that you should choose a suitably fast and tough fish for this, otherwise the tankmate can become the feeder.
Another way to let your predator fish recognise pellets as food is to go step by step. It is a big jump from feeder fish to pellets because the two food does not look similar not taste similar. To go step by step
1. Feed your fish with feeder fish plus some feeder shrimps.
2. Feed mainly feeder shrimps.
3. Feed feeder shrimps plus some market prawns. (if your fish does not eat the market prawns you should decrease total food so your fish is a little hungry. Hunger opens up the mind. In fact if hungry enough, even humans get less choosy on food)
4. Feed mainly market prawns.
5. Feed market prawns with pellets (yes stuff them in the prawn meat and make sure you stuff it in the middle. This is to let your fish get used to the pellet's taste.
6. Feed market prawns(with pellets stuffing) and pellets.
7. Feed pellets.