How to encode CPQ RichText attribute to Base64 as not accessible in any functions

Summary:

We have a requirement to send CPQ Rich Text attribute values to Enterprise Contract to generate a contract, however in Enterprise Contract there is no Rich Text attribute and CLOB attribute is being used to capture the Rich Text value as Base64 format using SOAP Api provided as part of the CPQ implementation.
CPQ has an encodeBase64 function to encode String to Base64 format however i am unable to access Rich Text attributes in any of the CPQ library functions or Advance functions.

Any idea how this can be done? If encoding to Base64 can be achieved via XSL when transforming it to XML in SOAP if not in CPQ also can be helpful.