It appears that the custom dataclass creation module in SMP 7.x contains the option to set a dataclass to handle multi-row entries, however with my customer we could not get the tick box to effectively turn an existing data class to multi row.
So in the end we resorted to export/import with a manual edit of the xml (using notepad++) to set the multirow parameter to true instead of false.
I will try to replicate this as soon as I am back in the office, but right now I am wondering if during the design it was considered that updating dataclass should not be allowed (thus changing from single to multi-row or the opposite be ineffective any times after dataclass creation).
Having said that we tried to create a new dataclass but this failed for some unknown reasons (the dataclass never appeared on the custom dataclass section).
And a final word on the brief report from my customer: custom classes not created via the custom class wizard (imported for example)...