Unfortunately, the described scenario is impossible. To achieve a greater control over the way properties are edited, you have two options:
1. Create a designer for User Control.
2. Create a type editor for your property.
Create a designer for User Control
The designer is basically a completely customizable user interface for editing properties of a control. The pros of this approach are that you are not limited in any way how are you going to let your users edit properties (drop down list, grid view, tree view...), while cons are that it is a bit time consuming.
More info on designers for User Control as well as the sample designer you can find here:
Create a type editor for your property
Type editors can be thought of as designers for only one single property. When you implement a type editor, Sitefinity will display a little "Select" button next to the property text box (you may have seen this already in some of the controls).
More information on implementing a type editor you can find here:
I hope you'll find this information helpful. Let us know if there is anything else we can do for you.
All the best,
the Telerik team