In the file /usr/local/pkg/tftp.inc add -w and -s options so it looks like this:

/* add tftp daemon */
fwrite($inetd_fd, "tftp\t\tdgram\tudp\twait\t\troot\t/usr/libexec/tftpd\ttftpd -w -s /tftpboot\n");

and you will be able to write new files into the /tftpboot directory of the server!