In general, it is easy to add items into your scope at your next scheduled surveillance or renewal audit. At a minimum, the auditor will review a design package for each item that you want to add to scope, unless the items are exempt per API Advisory 6. They also want to ensure that you have capability to manufacture conforming products within the new scope. API has recently issued an advisory that details what is required:
https://www.api.org/-/media/Files/Certification/Mo...

I don't know what is involved if you want to add an item prior to your next audit. There is a good chance that it will require a supplementary audit, although maybe you can convince them to let you add something similar to your existing scope, especially if it does not require design (i.e. it is listed on API Advisory 6). However, if your current API 16A license scope only includes adapters and drilling spools, I would be very surprised if API lets you add ram BOPs without them first auditing a ram BOP design package and relevant manufacturing capability.