Thanks for your question, Ricky. Yes, it is possible to modify library blocks, but we urge caution when doing so. I'd recommend you work with a local copy of the sub-library you want to modify instead of the original source. Otherwise, you will effectively be corrupting the original source library that is used in several of the reference applications in Powertrain Blockset. This will also make it difficult to run MATLAB updates in the future if your install directory has been modified.
These libraries are nested, so you have to unlock the library you want to modify along with its parents libraries. After unlocking a library, perform and "update diagram" (Ctrl+D), then right-click the block to disable the link.
Once you're done with your modification, make sure you test it thoroughly. There are several scripts running in the block masks and the reference applications that use the blocks. Making changes to the library blocks can have unintended side effects.