digital signature – Generate an SHA512/256 MAC from an SHA512

I need to verify a MAC for something signed with HMAC-SHA512/256, however, the language I’m using (Salesforce scripting) doesn’t have support for that algorithm. It does have HMAC-SHA512 and HMAC-SHA256 though – is there a way to transform one into the other, or somehow emulate the truncated version via other arithmetic/string operations?