You can, I've already made a modified version of Apollo and sideloaded it onto my phone with a tweak to set my own key instead of Apollo's

However there is a line on the developers page now that says all apps need to accept terms and sign up for a developers program, so I have my doubts it will continue to work either

The developer of Apollo has no interest in working with Reddit after this cluster fuck.

The API TOS has some nebulous verbiage about doing things that let a developer avoid the per app quotas, I'm sure reddit would apply it to any app that publicly did this

Please don’t do this, it’ll just make it even harder for actual developers and moderators to use Reddit’s API if people start abusing it.