Use a viewScope e.g. viewScope.selectItems variable. 
- Use this viewScope as the selectItems list. 
 
- Add the initial values to it.
 
- Later, add a additional new item to this viewScope and then it will appear in combobox's selection item list. 
 
This is a working example:
<xp:comboBox
    id="comboBox1"
    value="#{sessionScope.test}">
    <xp:selectItems>
        <xp:this.value><![CDATA[#{javascript:
        if (!viewScope.selectItems) {
            viewScope.selectItems = ["your","initial","values"];
        }
        return viewScope.selectItems;}]]></xp:this.value>
    </xp:selectItems>
</xp:comboBox>
<xp:inputText
    id="inputText1"
    value="#{viewScope.newItem}">
</xp:inputText>
<xp:button
    value="Add to selectItems"
    id="button1">
    <xp:eventHandler
        event="onclick"
        submit="true"
        refreshMode="complete">
        <xp:this.action><![CDATA[#{javascript:
            viewScope.selectItems.add(viewScope.newItem); 
            viewScope.newItem = "";
        }]]></xp:this.action>
    </xp:eventHandler>
</xp:button>