I’ve got some images to better illustrate what you can and cannot do and hopefully help you out.
In this image I’ve created a simple BSP/Geometry box. With the BSP’s the pivot point that controls placement/scaling/rotation is centered to the given brush that you select from the placement mode in the top left when creating a BSP.
This means that when I choose to scale/rotate/move an object it is doing so based on this pivot point. In terms of what you’re looking to do with scaling the Y-axis. you can see that I’ve highlighted the Y-axis and when I adjust it’s scale upwards the brush bounds extended outward in both directions away from the pivot point equally.
In contrast to this, I’ve grabbed a floor static mesh that was made in a 3d software package and imported into the UE4.
In the first image you’ll notice the pivot point is not centered and is in the bottom left corner.
In the second image you’ll see that when I scale the Y-axis upwards it is scaling the wall out away from the pivot point. The problem you may notice between the first image and the second image is that the texture is now stretched. Because this is a static mesh and not a BSP the UV coordinates will be stretched like this.
In this image with the BSP/Geometry box brush if you select the highlighted red box in the upper left corner you will be able to edit your geometry vertices individually. You can highlight the four vertices on the right wall as I have done and highlighted and drag them along the Y-axis. that you will see in the image below this one. In the bottom image you will also notice that the texture is not being stretched as it was with static mesh that was imported. Another note to point out is that this editing vertices tool cannot be used on any static meshes as it is only able to be used with any of the BPS/Geometry brushes.
Let me know if this helps you out or if you have further questions I’ll be glad to help you.