first bit of advice i can give, if youre using debian then try not to alter the config files that debian provide. work with them and add new ones, then when you do an apt-get upgrade you don't get asked about them!
so i'd create a new transport
probably looking something like...
driver = appendfile\n user=yourusername\n group=yourgroup\n mode=0600\n directory=/home/username/Maildir
you might want create_directory = true in there to autocreate the maildir
maildir_format = true will be needed too