Can one have both of these diseases: addison's and late onset cah?

Yes. They affect the adrenal gland by different mechanisms.
No not possible. Congenital adrenal hyperplasia is a congenital autosomal recessive disorder due to deficiency of one of the enzymes like 21 hydroxylase resulting in increase amount of androgens.And addison's disease is an aquired disease of adrenals caused due to auto immunity or infections and as a result adrenal is not able to produce cortisone and it can lead to crises if not treated effectively.You can not.