You don't say anything 'bout your Progress version - which can make a dramatic difference when adding fields to the schema pre-V9 as the fast schema change feature was introduced with Progress V9 ...
You also mention the 2GB limit which is an restriction of ancient and outdated Progress version. The limit was lifted, I think, somewhere around V9.1c where you could enable large files ...
Adding new indexed fields or new indexes to a table will be done in one transaction. The BI file holds the notes to undo the transaction in case of a crash. Therefore, if the table contains lots of records it might very well be that you hit the 2GB limit and as soon as the database can't write to the BI anymore it automatically crashes.
Regards, RealHeavyDude.