Sure, nothing could be easier

Just generate it from decimal 58.5 or HEX 3a.8  

Jokes aside - if you want to derive a private key from a floating-point decimal for experimental or educational purposes, you could devise your own method. Bitcoin private key from a floating-point decimal isn't standard practice, as Bitcoin private keys are typically generated using cryptographic algorithms that rely on random numbers.

good luck in whatever you're trying to achieve