Yes, of course, you can add your own libraries, and you should be very pleased to do so. If your library is publicly available, you can simply register it at packagist.org. If not, it is a little more difficult, but not impossible.
If your project does not comply with the PSR-0 standard, the composer will create a class map for you. Custom autoloader is not supported.
I would recommend you read the (really excellent) documentation about all this and come back if you have problems.
http://getcomposer.org/doc/
Alexander M. Turek
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