Creating external tables from SQL Server using JDBC Drivers
All I need is to create an external table from MSSQL using JDBC drivers. Is this possible?

No.  The supported data sources are here:

Hadoop
Azure Blob Storage
SQL Server
Oracle
Teradata
MongoDB
ODBC Generic Types

https://docs.microsoft.com/en-us/sql/relational-databases/polybase/polybase-guide?view=sql-server-ver15