Add missing rows for cstm tables

Hi everyone

I have added in a new custom field for a custom module at a later date after it was deployed. This module never had any custom fields before and now has a new one so the _cstm table is created. My issue with this is it only has rows for records that have since been updated. 

mysql> SELECT COUNT(*) FROM dvjxn_dealership_vehicle_jxn_cstm;
+----------+
| COUNT(*) |
+----------+
| 21958 |
+----------+
1 row in set (0.01 sec)

mysql> SELECT COUNT(*) FROM dvjxn_dealership_vehicle_jxn;
+----------+
| COUNT(*) |
+----------+
| 265096 |
+----------+

I wish to do a mysql update query for the cstm table but it does not have all the matching rows from the main table. Is there a way I can create these rows so I can run a query to update the custom field on _cstm table?