GOOD QUESTION !
For such operations, you have to deal with more than one table at a time.
ON BOTH SIDES (SAGE and OpenERP)
1_ You have to identify the DATA you need to migrate
2_ You have to identify, in which tables, and which columns, those DATA are recorded.
most often, if not always, the data in one table had to be spread on more than one table.
3_ How to deal with Foreign Keys and constraints ?
This is a big job. The kind of job that each computer scientist must do at least one time in his life.
You can find many responses to your questions in the following links:
dreis open-erp-data-integration20130704
Terminatooor
Anybox migration
Good luck, and keep us informed of the work progress.