You can scale numerical variables by using transformations in PMML. These can be part of the TransformationDictionary or LocalTransformations elements. For neural networks, numerical transformations can also be done in the NeuralInputs element (as well as the NeuralOutputs element).
For example, the transformation element NormContinuous can be used to implement simple normalization functions such as the z-score transformation (X - m ) / s, where m is the mean value and s is the standard deviation.
Pleaser, refer to the transformations page of the dmg website for PMML examples.