May be worth to post this question in the ePDM branch rather than API branch (Dispatch add-in is not really API). You definitely can do this with native API (both ePDM and SolidWorks). You need to read an existing value (IEdmVariableValue5::GetValue or CustomPropertyManager::Get3). Append your new data and reset the value.
Regards, Artem Taturevych | Snr. Developer | IC3D ANZ
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