> I am sure it is possible but will this "break" the certification and make that copy
> of X-Payments non-certified?
PA-DSS certification is granted to certain state of the software. If you change it there are several ways after that depending on size of change -
http://blog.halock.com/blog/security-sense/what-is-a-pa-dss-minor-change
Adding a new payment gateway integration to a PA-DSS certified software leads to re-assessment of the change by a PA-QSA. Usually such re-assessment costs a fee (may be measured in hundreds of USD or more), but it is much cheaper than re-assessment of entire software.