RadFilter programmatic creation is straightforward and could be done in the Page_Init event where the FieldEditors should also be created as shown below.
However, populating custom controls inside expression items could not be achieved in ExpressionItemCreated event. You will need to create a custom field editors which properly extract and copy their values. You could go trough the help article below which describes how custom field editors could be implemented.
If your issue still persist you could open a formal ticket and attached a sample project
showing the unwanted behavior so we could debug it and advice you with the best possible solution.
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