adding value to lookup multiple select field

I have multiple select drop-down lookup field and under it I created input field for adding values to select. I am using SPServices to add new value to lookup-list and so far everything is working good. The problem is how can I append added value to multiple-select. I tried to append result to selected, it shows there fine, but after saving form, it's not saved. I guess I need to add new value somewhere in object model, can someone give me hint how to do that.