Operating system name and version: FreeBSD 9.0 amd64
Version of smsd: smstools-3.1.14
Smsd installed from: FreeBSD ports tree
Name and model of a modem / phone: Huawei e1550
Interface: USB

How can I send sms via network from local machine to remote server that runs smsd with modem?

You need some way to transfer text files.
One solution would be ftp.
Another would be SMB,NFS
and another would be some php script and apache, that reads the file remotely over http and creates the files local on disk, inside the proper folders.