Can you do this in Slade 3?

What exactly you would like to do? In case you want to create window with sky behind, you don't need to use it as a wall texture.

I want to do something like... I don't know.. Oh wait! Like in hellonearthstarterpack.wad, here, let me show you, I want to put the F_SKY1 on the wall, so he looks like this:

Yes! I want it look like a filed. sry my english again

lol i wanted to say field XD

Hope I will find some answers till I'l get back from grandma.

use Line_Horizon Linedef effect (assuming you're making your level for ZDoom/GZDoom).

In case you are using Vanilla, you can do this:
1) Make a window sector (for example ceiling 128 high, floor 64 high).
2) Make outside sector behind the window with F_SKY1 texture on ceiling (ceiling 160 high, floor 0 high).
3) Make sector, where the horizon should be with the F_SKY1 texture ceiling. It should have 0 height on the floor level of outside sector (ceiling 0 high. floor 0 high).

4) Keep the upper wall texture between outside sector and horizon sector without any texture.

Will you please make a screenshot if you want?