Auto Populate CheckboxList In WFFM

You can write your own rule in Sitecore WFFM.

public class AutoPopulateCheckBoxListOption<T> : RuleAction<T> where T : ConditionalRuleContext
    {
        public override void Apply(T ruleContext)
        {
            Item FieldItem = null;
            if (ruleContext.Control is IResult)
            {
                FieldItem = GetItemFromDatabase(Sitecore.Context.Database.Name, ((IResult)ruleContext.Control).FieldID);
                if (FieldItem != null)
                {
                    var cblType = (Sitecore.Form.Web.UI.Controls.CheckboxList)(IResult)ruleContext.Control;

                    string fieldValues = "A::B::C::D";

                    if (!string.IsNullOrWhiteSpace(fieldValues))
                    {
                        string[] values = fieldValues.Split(new[] { "::" }, StringSplitOptions.None);
                        foreach (string c in values)
                        {
                            cblType.Items.FindByValue(c).Selected = true;
                        }
                    }
                }
                return;
            }
        }

        public Item GetItemFromDatabase(string database, string FormUri)
        {
            Sitecore.Data.Database db = Sitecore.Configuration.Factory.GetDatabase(database);
            return db.GetItem(FormUri);
        }

    }