Converting Non-Humanoid Animations into Humanoid Animations

So,

I have animations made for a custom rig but the problem is, that they don’t play if the humanoid is inside the character. But, the custom rig does follow a R15 rig. So my question is; do I have to fully re-animate the character with a humanoid or is there a way to convert the animation into one that is playable with the humanoid?