If you can get the sprites for the preset character you want (either rip it yourself or join the discord and someone there can help you), you can make a new Custom Player Character with the preset sprites, then use the remade Custom PC as a base for your Custom NPC. You may also need to change the name for this kind of NPC though.
But I suspect you're really asking if the dev can make the preset Player Characters selectable for Custom NPCs when you enter their names into the Custom Player Character Definition Name field.