You can check out both e-PatternsCentral.com or
[email protected]. They have a mix of both quilt and clothing patterns...might also google sweatshirts.com Amma making patterns from clothes is a standard practice in seamworking...there should be no problem. I would advise using muslin as your pattern material...it would be more durable ,you won't have to risk cutting the original, and you will have a permanent sweatshirt pattern to work from.