<https://forums.geocaching.com/GC/index.php?/topic/272463-can-you-bulk-add-to-bookmark-list/#comment-4685396>
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"If I am understanding your question and if their list is public you should be able to make a PQ of it and a PQ of yours and combine the two GPX files. \n"
<https://forums.geocaching.com/GC/index.php?/topic/272463-can-you-bulk-add-to-bookmark-list/>