You have to make sure you don't have too many sts for the dbl pointed needles or they'll fall off the ends. You can pick up the sts with a dpn, then slip them onto the circular. You don't have to decrease the sts, you can have just a straight bottom instead of a curved one, but you need to close up the bottom some way. You can leave the sts on the needle and do a three needle bindoff, or kitchener the sts together, or bind them off and sew it closed. There are videos for all these things under Advanced Techniques, Tips and Cast off.
sue- knitting heretic