In asymmetric cryptography (aka public-key cryptography) using Elliptic Curves or using Rivest–Shamir–Adleman algorithms, the private key is simply a number that we select at random in a certain range.
So to answer your question, if your RSA private key is selected at random and is in range to be a valid bitcoin private key (256 bit and between 1 and N) then you can use that "number" as your bitcoin private key.
But the issue is that since a 256 bit EC curve provides the same security as a 3072 bit RSA key and a 256-bit RSA keys aren't usually found, you can't convert the two. In simple terms there is a good chance that the RSA key you have is so much bigger than 256 bit.
That thread says you can use the RSA private key to generate a bitcoin private key by putting it through sha256. There's no good reason to do this though.
Sha256 takes n > 1 bits as input and gives a 256 bit output. Since its deterministic you will always get the same output, given the same input. That being said, it's better to just randomly generate a sequence of 256 bits and use that instead.
If the input is random the output can be considered random too. Assuming the RSA key was selected randomly SHA256 of it is secure enough to be used as a private key assuming it is a valid value (within range).
The whole process is pointless though, since you can simply select a random bitcoin private key instead of selecting a random RSA key then hashing it. This is also risky since if you leak the RSA key you'd also leak the bitcoin key.
