I have previously used NetSuite's Internal ID to create account mapping transformation rules. It was basically a set of transformation rules where source was the internal ID and target was account members in OneStream.
It worked well for us because the internal ID for each account in NetSuite was unique and there were no duplicates. I would assume as long as your external ID are unique and there are no duplicates, you would be able to create transformation rules in OneStream.
Things to make sure:
1) Make sure the external ID is unique for each OS member equivalent
2) Make sure all records do have an external ID assigned.
Hope this helps.
