Fluent NHibernate Mapping Entity From Multiple Columns

I have a legacy database that I am trying to create a mapping from that pulls from two tables (Table_A, Table_B) to create a Module with a list of Lockers. The key problem is creating a domain object from the Locker_Sizes and Locker_Top_Positions columns. I have attempted to map them with the IUserType below, but have no luck. Is it possible to do this mapping with Fluent NHibernate?

Module Mapping

public ModuleMap()
    {
        ReadOnly();
        Table("Table_B");
        Id(x => x.PartNumber).Column("ID_ITEM");
        Map(x => x.WorkOrder).Column("ID_SO");
        Map(x => x.SerialNumber).Column("Serial");

        Join("Table_A", m =>
        {
            m.KeyColumn("ID_ITEM");
            m.Map(x => x.LockerList)
               .Columns.Add("Locker_Sizes","Locker_Top_Positions")
               .CustomType<LockerString>();
            m.Map(x => x.IsBeepEnabled).Column("Is_Kiosk");
        });
    } 

Tables

CREATE TABLE `Table_A` (
`ID_ITEM`   TEXT,
`Is_Kiosk`  TEXT,
`Locker_Sizes`  TEXT,
`Locker_Top_Positions`  TEXT,
PRIMARY KEY(ID_ITEM)
);

CREATE TABLE `Table_B` (
`ID_ITEM`   TEXT,
`ID_SO` TEXT,
`Serial`    TEXT
);

Query

var modules = session.CreateCriteria(typeof(Module)).List<Module>();

LockerString Conversion

 public object NullSafeGet(IDataReader rs, string[] names, object owner)
    {
        // get as string and return list of lockers
        // names[0] Locker_Sizes "5,5,5"
        // names[0] Locker_Top_Positions "1,6,11"
        var lockerSizes = NHibernateUtil.String.NullSafeGet(rs, names[0]) as string;
        var lockerTopPositions = NHibernateUtil.String.NullSafeGet(rs, names[1]) as string;
        List<Locker> lockerList = new List<Locker>();
        var sizesTok = lockerSizes.Split(',');
        var topPositionsTok = lockerTopPositions.Split(',');
        for (int i = 0; i < sizesTok.Length; i++)
        {
            var locker = new Locker(int.Parse(sizesTok[i]), int.Parse(topPositionsTok[i]));
            lockerList.Add(locker);
        }
        return lockerList;
    }

Domain Objects

public class Module
{
    public virtual string WorkOrder { get; set; }
    public virtual string PartNumber { get; set; }
    public virtual string SerialNumber { get; set; }
    public virtual string IsBeepEnabled { get; set; }
    public virtual List<Locker> LockerList { get; set; }
}

public class Locker
{
    public virtual int Height { get; set; }
    public virtual int TopPosition { get; set; }

    public Locker(int _height, int _topPosition)
    {
        this.Height = _height;
        this.TopPosition = _topPosition;
    }
}