Question about modifying my FILSTAR XP-S Filter

[KMS.Kyle]

Hello - I purchased my filter used but fairly new and noticed that the previous owners of it had placed a bunch of smaller pieces of live rock into the filter - I'm assuming to act as sort of a sump to a degree?

I was wondering if I could safely add anything else to it? like possibly that filter cotton or something to give it some extra filtering.

Thank you