This would be really useful functionality. We often need to implement this as we have many domain classes with Lists of doubles / ints. 

Following Ticket 1657199 here is one way to acheive this functionality.

It will provide a value type Add command for all collections which inherit from IList.  

In your code behind call:

this.radPropertyGrid.AutoGeneratingPropertyDefinition += ValueTypeAddCommandProvider.AutoGeneratePropertyDefinition;

And add this to your codebase:

using System;
using System.Collections;
using System.Windows;
using System.Windows.Markup;
using Telerik.Windows.Controls;
using Telerik.Windows.Controls.Data.CollectionNavigator;
using Telerik.Windows.Controls.Data.PropertyGrid;

namespace _1657199;

public class ValueTypeAddCommandProvider : CollectionNavigatorBaseCommandProvider {
    public ValueTypeAddCommandProvider() : base(null) { }

    public ValueTypeAddCommandProvider(CollectionNavigatorBase collectionEditor) : base(collectionEditor) {
        this.CollectionNavigator = collectionEditor;
    }

    public override void AddNew() {
        if (CollectionNavigator.Source is IList list) {
            var t = this.CollectionNavigator.Source.GetType();
            if (t.IsGenericType && t.GenericTypeArguments.Length == 1 && t.GenericTypeArguments[0].IsValueType) {
                var listType = t.GenericTypeArguments[0];
                    
                var newItem = Activator.CreateInstance(listType);
                if (newItem != null) {
                    list.Add(newItem);
                }
            }
        }
            
    }

    public override bool CanAddNewExecute() => true;

    public static void AutoGeneratePropertyDefinition(object sender, AutoGeneratingPropertyDefinitionEventArgs e)
    {
        var descriptor = e.PropertyDefinition.PropertyDescriptor;
        var isPrimitiveList = descriptor is { PropertyType: { IsGenericType: true, GenericTypeArguments.Length: 1 } } &&
                              descriptor.PropertyType.IsAssignableTo(typeof(IList)) &&
                              descriptor.PropertyType.GenericTypeArguments[0].IsValueType;
        if (isPrimitiveList) {
            Type editorType = typeof(CollectionEditorPicker);
            Type commandType = typeof(ValueTypeAddCommandProvider);
            var parserContext = new ParserContext();
            parserContext.XmlnsDictionary.Add("", "http://schemas.microsoft.com/winfx/2006/xaml/presentation");
            parserContext.XmlnsDictionary.Add("myTelerik", $"clr-namespace:{editorType.Namespace};assembly={editorType.Assembly.GetName().Name}");
            parserContext.XmlnsDictionary.Add("myCommand", $"clr-namespace:{commandType.Namespace};assembly={commandType.Assembly.GetName().Name}");
            e.PropertyDefinition.EditorTemplate = XamlReader.Parse("<DataTemplate>" +
                                                                    $" <myTelerik:{nameof(CollectionEditorPicker)} {nameof(CollectionEditorPicker.Source)}=\"{{Binding Path={descriptor.Name}}}\"> " +
                                                                        $"<myTelerik:{nameof(CollectionEditorPicker)}.{nameof(CollectionEditorPicker.CommandProvider)}>" +
                                                                            $"<myCommand:{nameof(ValueTypeAddCommandProvider)}/>" +
                                                                        $"</myTelerik:{nameof(CollectionEditorPicker)}.{nameof(CollectionEditorPicker.CommandProvider)}>" +
                                                                    $"</myTelerik:{nameof(CollectionEditorPicker)}>" +
                                                                   "</DataTemplate>", parserContext) as DataTemplate;
        }
    }
}