Is there a way to add more ECTS to my degree?
Adding ECTS to your degree after you graduated is probably not possible.

However at least some European universities will let you enrol to a single course/module: you can attend one or more courses without being enrolled for a degree and earn a certificate for the ECTS you need.
You are after some generic not-too-advanced Mathematics/Statistics training which you can find in many scientific (Biology?) degrees at most institutions, so it should be quite easy to find something suitable for you.

For example you can find information about this at University of Milan or London.

Depending on the circumstances this could be expensive, demanding and takes some time (if you really need the credits before applying you need to wait at least one semester, possibly one year), so it is probably better to contact the department offering the MSc you are interested in to try to decide whether it's worth doing.