Hello: Here in the USA many new laws are being proposed to ban the import of non native (exotic) species of fish, reptiles, plants and such. Some non native species have been released into the environment and have been causing serious problems where they have been able to reproduce and impact the native wildlife.
People that do not want species banned for import often cite that species from warm climates cannot survive in colder climates. Your experience seems to question this assumption. However the fact that an adult can survive the winter in a pond that freezes over does not always indicate that species can raise offspring under those conditions. Survival of an adult is a good indicator that this may be possible.
Raising fry would very well be possible, since summers in mainland europe ( except scandinavia) are very warm. The question is if those fry would survive the winter.