[SalesForce] Adding Multiple Products to Related list of custom object

I am building CPQ Module for my client and I am done with Product and Bundle management
Now, I am implementing Quote management and I am stuck for the following :

I am developing one visualforce page and apex class for the following requirement

I have a custom object named QBP__c and in this object I need to store multiple Products in related list (only existing products) with entering Quantity of each products

To achieve this requirement I have created one field Product__c(Lookup) because user will select product using this lookup field and after clicking on the save button it will
move to the related list of the QBP__c object

Now, the question is how can I add multiple Products to this related list from selecting the product using lookup field ? And How can I add quantity to each product while saving from visualforce page ?

Let me show you my visualforce code :

<apex:pageBlockButtons location="Bottom">
    <apex:commandButton value="Add Product" action="{!addSection1}"/>
    <apex:commandButton value="Save" action="{!Save}"/>
    <apex:commandButton value="Cancel" action="{!Cancel}"/>
</apex:pageBlockButtons>
    <apex:outputPanel >
    <div>
    <ul id="sortable">
     <apex:variable value="" var="foo" rendered="{!IF(lstSections1 = True, true, false)}">
            <apex:pageBlockSection title="Select Product">
                <apex:actionRegion >
                    <apex:pageBlockTable value="{!QBP}" var="QBP">
                    <apex:column >
                        <apex:outputLabel >Select Product :</apex:outputLabel>
                    </apex:column>
                    <apex:column >
                    <apex:inputField value="{!QBP.Product__c}" >
                        <apex:actionSupport event="onchange" rerender="relatedInfo" action="{!retrieveproductInfo}" immediate="false"/>
                    </apex:inputField>
                    </apex:column>
                    <apex:column >
                        <apex:pageBlockSection id="relatedInfo">
                            <apex:outputText value="{!partNumber}" label="Part Number :"/>
                            <apex:outputText value="{!Description}" label="Description :"/>
                        </apex:pageBlockSection>
                    </apex:column>
                    </apex:pageBlockTable>
                </apex:actionRegion>
            </apex:pageBlockSection>
    </apex:variable>
    </ul>
    </div>        
    </apex:outputPanel>

Apex Class :

    public class AddQBPForQuoteController{

    public Quote quote{get;set;}
    public boolean lstSections1{get;set;}
    public String partNumber{get;set;}
    public String Description{get;set;}
    public QBP__c QBP{get;set;}
    public List<QBP__c> QBPList{get;set;}

    public AddGroupForQuoteController(ApexPages.StandardController controller) {
        lstSections1 = false;
        grp = new Group__c();
        QBP = new QBP__c();
        QBPList = new List<QBP__c>();
        this.quote = (Quote)controller.getRecord();
    }

    public void retrieveproductInfo() {
        if(QBP.Product__c != Null){
            this.partNumber = [SELECT Id, ProductCode FROM Product2 WHERE Id =: QBP.Product__c].ProductCode;
            this.Description = [SELECT Id, Description FROM Product2 WHERE Id =: QBP.Product__c].Description;
        }
    }

    public PageReference addSection1() {
        lstSections1 = true;
        QBP = new QBP__c();
        QBPList.add(QBP);
        return null;
    }

    public Pagereference Save(){
        insert QBP;
        for(integer i=0; i<QBPList.size(); i++){
            this.partNumber = [SELECT Id, ProductCode FROM Product2 WHERE Id =: QBP.Product__c].ProductCode;
            this.Description = [SELECT Id, Description FROM Product2 WHERE Id =: QBP.Product__c].Description;
        }
        upsert QBP;
        return null;
    }

    public Pagereference Cancel(){
        PageReference pg = new PageReference('/apex/AddQBPForQuote?id='+quote.Id);
        pg.setRedirect(True);
        return pg;
    }

}