Mannually edited data in the persistence.xml are overwrote

After generate persistence mapping from database source the "persistence.xml" was created. I had manually set

<property name="hibernate.connection.username" value="username"/>

<property name="hibernate.connection.password" value="userpassword"/>

and everything work fine. But when a new persistence mapping is generating this values are overwrote and 
become empty. It's necessary to set them again. Are there any options for set these values from a
data source properties or prevent overwriting after each mapping generation?