A possibility, may not be a very good one, is to use either a User-defined Java Class or Java Expression step to covert the text to an array of bytes and then create the sha256sum hash on that. I haven't tried this, but if you specify String.getBytes( "UTF-16LE" ) that should get you the input you need to the sha256sum algorithm. Now, will that work with the MessageDigest.digest method? I'm not at all sure. This algorithm may choke on the 0x00 bytes you'll have in the UTF-16LE byte array. All the info I found in a quick search shows converting to UTF-8 before calling the digest method. See for example: https://www.baeldung.com/sha-256-hashing-java
But, that may just be a convention. I'm sorry not to take time to test is out, but I'll leave that to you.
Hope this may help!