In CorelDRAW, an outline is only allowed to have a solid color. If you want something fancy like a fountain fill, you can’t apply it to an outline.
But this is CorelDRAW and there is a way to do almost anything. Select the object with the outline you wish to fill and select Arrange | Convert Outline to Object or simply press Ctrl + Shift + Q and the outline will now be a fillable object. In fact, you could even add an outline to that object which gives you the effect of multiple outlines. This is a very basic feature, but it packs a very powerful punch in a number of different ways. Put it to use in your projects!