The problem statement seems to be that you have local values that are considered 'source code' that are longer than varchar(50). Although you could assign 2 billion + concept_id for these, the long source code value gets truncated.
Some options - not necessarily in a recommended order
1. Change the DDL of the omop concept table. Increase varchar(50) to varchar(255). Simplest, but you may have performance issues and most impact to OMOP vocabulary tables.
2. Use your source_code as concept_name, and populate source_code with random characters. You should have the same functionality, without changing DD, no impact to OMOP vocabulary tables - but you will have problems if > varchar(255)
3. Creatively use this table with some change to DDL for source_code, source_code_description etc. Probably most complex, but least impact to OMOP vocabulary tables