hi samantha and welcome to FF.
legally, you can alter any filter however you choose. you can even submit those to the library. however, the folks at FF have the final say on what is accepted to the library and some filters have been rejected for being too close to another, thus being a 'clone'.
there is also the non-legal and non-FF considerations. no one likes plaigerism/cloning/ripping off of another's ideas and products. however, there is that notion of 'i've got an idea of how to make this better or different' and that is actually a valid thing. the trick is how to present that idea/product without ripping off or offending someone else, particularly the originator. you've already taken the first step of avoiding that, asking

and we/
i do appreciate that.
so, what i normally do when i mod someone's filter is, let them know in here in the forums, check their filter to see if it's gotten a 'high use' (HU) rating and won them a reward because i dont want to pull usage away from their filter if they havent gotten an HU yet and talk to the originator about what they'd like me to do or not. and i rarely add a modded filter to the library, the exception being my 'silver surfer' filter. so, i normally, if the filter has gotten an HU already, post it here in the forums for others to grab.
if their filter hasnt gotten an HU yet, i let the creator know and ask what they'd like me to do with it. this sometimes results in doing nothing and i simply keep the filter to myself for my own use, or, i may send the mod to the original filter creator and let them submit the mod if they want and maybe ask for a nod in the description.
now, none of that is 'official' policy; it's just sort of what i try to do.
i havent seen darth axel around in quite a while, so i dont know if you'll be able to carry some of what i suggested out. so, not real sure what you can do here; maybe send him an email? the filter already has an HU, so you wont be taking a reward point away from him. that sort of leaves posting the filter here or emailing darth or asking the FF folks in the contact form.
good luck

If wishes were horses... there'd be a whole lot of horse crap to clean up!
Craig