You can connect the DBMan to an existing database ( when you install it or from Admin - Setup). It should overwrite dbman's tables (Dbsql_* and Sample_*) if you chose 'overwrite' option.
Also, you can load the existing tables:
- Current database:
+ Goto Admin - Tools - Create table
+ Put the name of table which is already exist in current database, DBMan will load that table automatically.
- From another database: you can use Admin - Tools - Load Table - From an existing SQL table.