I think this is a phrase DL would be better to change or lose. It sounds stilted in English and less you're speaking in strictly the mathematical sense, and in that case the terms are wrong. Where I'm from we would say something like "she is factoring that in", or "she is taking that into consideration". To my ears "she adds that factor" sounds stilted.
This has many possible meanings. For example, it could be that she is a woman bringing a skill set. Or, she is adding the factors of a number in math class. Or she could be an element of unpredictability in a volatile situation.
If you google "suma ese factor." you find many many examples of usage.
I'm not a native speaker but I went looking for the differences between sumar and añadir and found these links helpful:
It seems sumar is related to numerical counting and añadir is to incorporate X into Y (like when to add an ingredient to a recipe)