As an employer, can I bear out-of-bundle costs for my employee?

Yes, that's possible.
Please note: by default, the costs outside the subscription bundle are added to your employee's invoice. You can adjust this per employee by setting a monthly out-of-bundle allowance. This allowance can partially or fully cover costs outside the normal subscription bundle (calling, surfing, texting). As an employer, you adjust this compensation via My Business, where you manage the subscriptions of your employees. The amount you set here (between € 0 and € 300) is then the maximum that you can be charged for out-of-bundle costs. If your employee uses less or nothing, this will of course not be charged. You will always see this on your monthly invoice.