Storing Objects in ItemStack Capability Storage

I am trying to store data through my ItemStack capability storage, but I stumbled upon a roadblock.

My ItemStack Capability Storage:

public class WandItemStorage implements Capability.IStorage<IWandCap>
{
    @Nullable
    @Override
    public INBT writeNBT(Capability<IWandCap> capability, IWandCap instance, Direction side)
    {
        CompoundNBT tag = new CompoundNBT();
        tag.put("spells", instance.getSpells());
        return tag;
    }

    @Override
    public void readNBT(Capability<IWandCap> capability, IWandCap instance, Direction side, INBT nbt)
    {
        
    }
}

My ItemStack Capability:

public class WandCap implements IWandCap
{
    private List<Spell> spells;

    public WandCap ()
    {
        spells = new ArrayList<>();
    }

    @Override
    public List<Spell> getSpells()
    {
        return this.spells;
    }

    @Override
    public void addSpells(Spell... spells)
    {
        this.spells.addAll(Arrays.asList(spells));
    }

    @Override
    public void addSpell(Spell spell)
    {
        this.spells.add(spell);
    }

    @Override
    public void removeSpells(Spell... spells)
    {
       this.spells.removeAll(Arrays.asList(spells));
    }

    @Override
    public void removeSpell(Spell spell)
    {
        this.spells.remove(spell);
    }

    @Override
    public boolean containsSpell(Spell spell)
    {
        return this.spells.contains(spell);
    }
}

I'm currently trying to put a List of Spells (my custom object) to the Write and Read NBT methods of my ItemStack cap storage, but CompoundNBT doesn't have a method for putting a custom object into a tag. All it has is the put() method from what I'm seeing, however the put() method requires an INBT for the second parameter, which I do not have. Would I need to convert my List of Spells to an INBT, and how would I go about doing that?