Implement a class that inherits NamingStrategy and do what you want. Here's the signature of it, quite straightforward(Note this code is Hibernate's one, but I guess it's quite close to this in NHibernate):
Code:
public interface NamingStrategy {
/**
* Return a table name for an entity class
* @param className the fully-qualified class name
* @return a table name
*/
public String classToTableName(String className);
/**
* Return a column name for a property path expression
* @param propertyName a property path
* @return a column name
*/
public String propertyToColumnName(String propertyName);
/**
* Alter the table name given in the mapping document
* @param tableName a table name
* @return a table name
*/
public String tableName(String tableName);
/**
* Alter the column name given in the mapping document
* @param columnName a column name
* @return a column name
*/
public String columnName(String columnName);
/**
* Return a collection table name ie an association having a join table
*
* @param ownerEntityTable owner side table name
* @param associatedEntityTable reverse side table name if any
* @param propertyName collection role
*/
public String collectionTableName(String ownerEntityTable, String associatedEntityTable, String propertyName);
/**
* Return the join key column name ie a FK column used in a JOINED strategy or for a secondary table
*
* @param joinedColumn joined column name (logical one) used to join with
* @param joinedTable joined table name (ie the referenced table) used to join with
*/
public String joinKeyColumnName(String joinedColumn, String joinedTable);
/**
* Return the foreign key column name for the given parameters
* @param propertyName the property name involved
* @param propertyTableName the property table name involved (logical one)
* @param referencedColumnName the referenced column name involved (logical one)
*/
public String foreignKeyColumnName(String propertyName, String propertyTableName, String referencedColumnName);
/**
* Return the logical column name used to refer to a column in the metadata
* (like index, unique constraints etc)
* A full bijection is required between logicalNames and physical ones
* logicalName have to be case insersitively unique for a given table
*
* @param columnName given column name if any
* @param propertyName property name of this column
*/
public String logicalColumnName(String columnName, String propertyName);
/**
* Returns the logical collection table name used to refer to a table in the mapping metadata
*
* @param tableName the metadata explicit name
* @param ownerEntityTable owner table entity table name (logical one)
* @param associatedEntityTable reverse side table name if any (logical one)
* @param propertyName collection role
*/
public String logicalCollectionTableName(String tableName, String ownerEntityTable, String associatedEntityTable, String propertyName);
/**
* Returns the logical foreign key column name used to refer to this column in the mapping metadata
*
* @param columnName given column name in the metadata if any
* @param propertyName property name
* @param referencedColumn referenced column name (logical one) in the join
*/
public String logicalCollectionColumnName(String columnName, String propertyName, String referencedColumn);
}