adding additional parameters to the submit_sm PDU

We've received a requirement from our SMSC that we need to add additional vendor specific SMPP parameters to our submit_sm PDU.  How would I defined these additional parameters in Kannel?  Is there something I can add to the request URL?  eg:

http://smsbox.host.name:13013/cgi-bin/sendsms?username=foo&password=bar&to=0123456&text=Hello+world&customVParam=test

Thanks!