I don't see how it would work from an animation standpoint, nor why it would be needed from a gameplay one. There's meant to be a trade off between crouching and running - one is slow and quiet, and the other fast but loud, which means that the player has to make meaningful choices about how they proceed. Crouched sprinting would be either an 'I win' button, or basically the same as simply walking. Which is a terrible idea in one case and useless in the other.
If you want to be quick and quiet, you can invest in those skills as you progress, which is still means you have to make a choice- points spent there can't be spent elsewhere. It's meaningiful and keeps it balanced.