In my experience, you will have appropriate isolation features between the safety and non-safety portion of your system (e.g., two safety-related fuses). If the modification is occurring entirely "downstream" of your isolation features and does not require changes to the design of the safety portion (I.e., no changes to the characteristics of the load that require your safety-related uninterruptable power system to be modified), then your changes should be non-safety-related.

Your local licensing engineer who is cognizant for Chapter 8 of your FSAR should also be able to help you make the call.

While your mod would need to be classified as a Safety Related modification, coincident with the highest safety classification of the components affected by the modification, that doesn't mean that every component added by your modification will need to be classified as safety related. You would need to follow all your procedures and ANSI N45.2.11 requirements for a Safety Related modification and evaluate your impact to the Safety Related components. You also need to make sure you're not violating anything from your original licensing basis, or you may need to go through the 50.59 evaluation process to determine whether you need NRC approval before making the change. Get Licensing involved early in the process.