SQL Function Call?//

Do you mean calling a SQL stored procedure passing an Xml document as a parameter?

If so, then you can do this with the SQL Adapter

Create your stored procedure:
CREATE PROCEDURE [dbo].[bts_PassXmlDocument] @XmlDoc ntext

Use the Add adapter metadata to create the schema for the stored procedure.

Promote the XmlDoc field from this schema as a Distinguished Field.

Generate a schema for the Xml document you wish to pass.

In an orchestration create a message for the SQL Stored Procedure schema (msgSQLMessage) and a message based on the Xml Document schema(msgXmlDoc) and declare an orchestration variable of type System.Xml.XmlDocument (varXmlDoc)

In a message assignment shape:

varXmlDoc = msgXmlDoc;
msgSQLMessage.bts_PassXmlDocument.XmlDoc = varXmlDoc.OuterXml;

Send the msgSQLMessage to SQL Adapter send port.