Hibernate version: 1.0.2
Name and version of the database you are using: Sql Server 2000

Thanks for your answer ...
The full solution is below

1) Indeed I create a new usertype :
------------------------------------------------------------------------------------
#region Inclusion des namespaces systèmes

using System;
using System.Data;

using NHibernate;
using NHibernate.SqlTypes;
using NHibernate.Type;
#endregion

#region Inclusion des namespaces applicatifs
using KPI.Mapping.Manag;
#endregion

namespace KPI.Mapping.Types
{
public class FormulaType : IUserType
{
StringType stringType = (StringType)NHibernateUtil.String;

public FormulaType()
{

}

#region IUserType Members

public SqlType[] SqlTypes
{
get
{
return new SqlType[] { stringType.SqlType };
}
}

public System.Type ReturnedType
{
get
{
return typeof(FormulaType);
}
}

bool IUserType.Equals(object x, object y)
{
if (x == y)
{
return true;
}
if ((x == null) || (y == null))
{
return false;
}
return x.Equals(y);
}

/// <summary>
/// Récupère la valeur dans la base de données
/// </summary>
public object NullSafeGet(System.Data.IDataReader rs, string[] names, object owner)
{
int index = rs.GetOrdinal(names[0]);

Formula loformula = new Formula();
if (rs.IsDBNull(index))
{
loformula.Value = null;
}
else
{
loformula.Value = rs[index].ToString();
}
return loformula;
}

/// <summary>
/// Passe la valeur à la base de données
/// </summary>
public void NullSafeSet(IDbCommand cmd, object value, int index)
{
if (value == null)
{
((IDbDataParameter)cmd.Parameters[index]).Value = DBNull.Value;
return;
}

string lsValue = ((Formula)value).Value;
if ( lsValue == null)
{
((IDbDataParameter)cmd.Parameters[index]).Value = DBNull.Value;
return;
}

((IDbDataParameter)cmd.Parameters[index]).Value = lsValue;
}

public object DeepCopy(object value)
{
return value;
}

public bool IsMutable
{
get
{
return true;
}
}

#endregion

} //Fin classe FormulaType
}

2 )This user type interfaces my Formula instance
------------------------------------------
private Formula _FormulaRange = new Formula();
/// <summary>
/// Formule de seuil :
/// formule dont le calcul permet de vérifier
/// que le résultat se trouve bien entre les limites inférieure et supérieure
/// du seuil
/// </summary>
public Formula FormulaRange
{
get { return _FormulaRange; }
set { _FormulaRange = value; }
}

3) And the Formula class
-------------------------------------------
namespace KPI.Mapping.Manag
{
public class Formula
{
private string _sValue = null;
public string Value
{
get {return _sValue;}
set {_sValue = value;}
}

public int mOwnFunctionalty()
{
//You do what you want
} //Fin mGetElements

} //Fin classe Formula
}