Ok now I understand your point.
So, a new Bank Statement format can not be created from runtime but it is possible from development point of view.
When a new Bank Statement import format is created in accordance with your Credit Card stmt format, then it should be possible for you to select that format and then upload. Extension of file doesn't matter as long as the data is readable by ByDesign system.
I've designed interface earlier for uploading CSV files into ByDesign. So, I know. 