This question should really be in the Masquerade subforum.
However, each con has different rules. Most cons just require that at least a certain percentage of the costume is made yourself for the workmanship/craftsmanship judging. Judges don't expect you to be able to make everything, especially if you're in a beginner or novice class.
However, if you perform a skit, you do not have to make any part of the costume yourself. You'll be judged on performance skill rather than creating skill.
Check the con's site for their rules and/or email the people in charge of the masquerade just to make sure that your costume is fine. Send pictures and tell them exactly what you're doing. If the answer is no, it's better to hear that before the con than get embarrassed in front of everyone in the green room.
My username is an incredibly stupid pun that only I will get.
Currently working on:
Momoko Koigakubo -- Ghost Stories (Dub)
Scarlett O'Hara Drapery Dress -- Gone With the Wind
Deandra -- MPGiS
Anna Cornonation Dress -- Frozen
Vader Eloha/The Queen -- OFF
Princess Suwa -- Mascot of the city of Suwa, Japan
My favorite cosplays:
Brittnay Matthews -- MPGiS
Mom Lalonde -- Homestuck
Princess Kenny -- South Park