As I understand it you are doing schema by schema exports of the old database and schema by schema imports into the new, 9i database and you want to know what mode the new 9i database should be in during the imports?

It should be open. If it is not open then imp will not run. I suppose it might be a good idea to open it in restricted mode until you have completed the imports and you may want to reserve a little time at the end of the migration to do a physical backup. If you do put it in restricted mode be sure to unrestrict it when you are done.