create_assignment function is not in web service of moodle

Hi...

I want to use create_assignment function of moodle for integrating it with openerp.

But in Site Administration->Plugins->Web Service->External Services->Add Functions,

there is not any function for creating assignment, it has function for moodle_assign_get_assignment.

but i wants to use create_assignment function.

So kindly help me out about this.