A question that is often explored with clients is the difference between the two. When is it better to do a customisation than modify a business process.
The question has to be analysed in two ways.
A business requirement gap should result in a customisation, though that does not mean that the full extent of the softwares functionality should not be extended by modifying the business process to then make the customisation as "least invasive" as possible.
Within an ERP system you should always look to maximise the benefits of the package and not write around functions with customisations that override core functionality as doing this will always mean you have a maintenance overhead as the software improves with newer versions from the vendor.
In short, yes you can do a customisation but it requires careful and full understanding of the full functionality to make the change and extend in the right place. You can only perform this after a full analysis by business personnel that are close to the business requirements. Allowing your technical teams to take charge of a business gap requirement invariably leads to suboptimal customisations.