Unfortunately, I was not able to find any documentation about the import behavior. So I invented a workaround.
I imported GraphML data set to Neo4j using the following node structure:
<node id="1">
<!--This code doesn't really force Neo4j to create PAGE label-->
<data key="label">PAGE</data>
</node>
Then I executed the following Cypher command:
MATCH (n)
WHERE n.label='PAGE'
SET n :PAGE
After that, Neo4j applied PAGE
label to each node that matches the search condition.
At the moment, this workaround is acceptable for me. It would be better to find the real solution, though (if any).