Thanks for considering Aspose.
And in the .cs file, you may create the validation
// Custom function
cell = cells["C13"];
cell.Validation.ClientValidationFunction = "myvalidation1";
For reference, please check the source code of Protection/Validation demo (DataValidation.aspx) in Common Demos category from the Aspose.Grid.Web featured demos: http://www.aspose.com/Products/Aspose.Grid/Demos/.
Also, is it possible to add custom function, like it can be done in Aspose.Cells with the ICustom interface?
Well, currently it is not available, we will support it in our future versions to extend the formula calculation engine.