No you cannot create your own ids for the system. You have to let the system create them it is actually safer that way. You can always export the data created to retrieve the ids afterwards. Leave the id column blank and OpenERP will assign ids to each line as needed

You can create your own External ID's for the system. This is exactly the reason the External ID field was created.

Map the field ontaining the ID you'd like to use to External_ID.

OpenERP will first check to see if a record with the same External ID exists. If it does, your import will be used to update a record. If it does not, your import will be used to create a record.

Refer to the documentation, and the help on the form where you import, for more direction.

https://doc.openerp.com/book/8/8_20_Config/8_20_Config_import_export/#importing-csv-data-to-openerp