You can create a new PersistenceManager. Then use ImportEntities to import the entities from one PersistenceManager to another.
Here is a code snippet which copies all changed Employees from one PersistenceManager to another.
mPmLegacy = new PersistenceManager(true, "Legacy");
mPmLegacy.ImportEntities(mPM.GetEntities<Employee>(DataRowState.Modified | DataRowState.Added | DataRowState.Deleted), MergeStrategy.PreserveChanges);