extbase mapping to an existing table doesn't work

Question

I've extended the pages table and now I want to use some of the data in a domain object called "Tags".
So I tried the following in the /Configuration/TypoScript/setup.txt:

plugin.myextension.persistence.classes.Tx_myextension_Domain_Model_Tag {
    mapping {
        tableName = pages
        recordType = Tx_myextension_Domain_Model_Tag
        columns {
            tx_myextension_tag_name.mapOnProperty = name
            uid.mapOnProperty = id
        }
    }
}

But It seems that the extension tries to access the table Tx_myextension_Domain_Model_Tag (which doesn't exist)

This is the error I receive:

Tx_Extbase_Persistence_Storage_Exception_SqlError`

Table 'tx_myextension_domain_model_tag' doesn't exist: SELECT tx_myextension_domain_model_tag.* FROM tx_myextension_domain_model_tag WHERE tx_myextension_domain_model_tag.id = '24' LIMIT 1

What have I done wrong?