In theory you can do anything you want, and in practice people do that too. As long as you don't break anything that #ActivityPub specs prescribe, the #ActivityStreams vocabulary is just a 'toolbox' of #SocialWeb primitives you can use as building blocks in your solution. The only issue arises if you want your mechanism to have #interoperability with other #fediverse solutions. They should either ignore, or understand it, and not break. Writing a #FEP is the best-practice way to walking the path of adoption of your approach.