Toad for Oracle: Procedures, Packages, Job, and DBMS_SCHEDULER
If so, can I call an existing script to create a procedure or would I
  need to paste the PLSQL into the procedure script?


Both works.

DBMS_SCHEDULER.CREATE_JOB

The type of the job (job_type) can be PLSQL_BLOCK or STORED_PROCEDURE as well.

For PLSQL_BLOCK, you can specify an anonymous block, a piece of PL/SQL code such as begin ... end;. 

For STORED_PROCEDURE, you can specify the name of the stored procedure to call.