Need magic for inserting in 2 tables

Hi,

I need to insert a lot of basically blank records into a table to be
filled later.
Sounds silly but please bear with me. :)

projects ( project_id, project_name, ... )
companies ( company_id, ... )
departments ( department_id, department )
staff ( staff_id SERIAL, company_fk, department_fk, ... )

company_2_project ( project_fk, company_fk )
staff_2_project ( project_fk, staff_fk, project data, ... )

So with this I can store that company 99 belongs e.g. to project 3, 5 and 42
and staff_id 11, 13, 17 belongs to company 99.

staff_2_project represents the connection of staff members to a project
and holds projectrelated infos.

Now say I have allready 100 companies out of the bigger adress pool
connected to project 42 and I now want to add blank staffers out of
department 40 and 50 linked with this project.

I do step 1:

insert into staff ( company_fk, ..., department_fk )
select company_fk, ..., department_fk
from departments, companies, company_2_project AS c2p
where company_id = c2p.company_fk
and c2p.project_fk = 42
and department_id in ( 40, 50 );

step 2 would be to link those new blank staff records to project 42 by
inserting a record into staff_2_project for every new staff_id.

How can I find the new staff_ids while making sure I don't insert ids
from other sessions?
Is there an elegant way in SQL ?