In the docs, it says that:

If a gene has its static space defined in the gene_space parameter, then mutation works by replacing the gene value by a value randomly selected from the gene space.

However, if a gene_space is defined as a dict with low and high parameters, it should be possible to generate additive mutations, so that new_value = old_value + random_value, rather than new_value = random_value.

I had a brief look in the source code, and this wasn't supported at the moment.