You just enter the subrace characteristics as part of the general race information. For example, Elves get +2 Dexterity and have subraces of Wood Elf (+1 Wisdom) or High Elf (+1 Intelligence). If I were making a Wood Elf character using the Custom Race option, I would combine both the traits from Elves, with the traits from Wood Elf. So I would put both the +2 bonus to Dexterity as well as the +1 bonus to Wisdom. As Stephanie points out, because you are using the custom race functionality, and the charactermancer custom settings apply only to the one character instead of every character, you don't need to specify a subrace for the custom race. As far as the charactermancer is concerned, the subrace is the main race of the character.