The newest and most complete work on dragonflies (Hansruedi Wildermuth/Andreas Martens: Taschenlexikon der Libelle Europas. Wiebelsheim: Quelle und Meyer 2014), 824 pages, ends with a chapter of the exotic dragonflies taht have appeared in Europe. Nearly all of them have been imported unintentionally by the trade with exotic plants for the aquarium. They mention 19 species of 15 genera.
I such a case as is told here the specialists would surely be happy about a note and – possibly – a photo of the larva. See the website http://www.libellula.org