I could not know the start and stop location until after first building the pond and then creating the Profile from Surface for the wall face.
If I am following your workflow correctly, you created a 3D Linear Element that has an unknown start/stop location until later in the design process. Your current workflow is to design the 3D Linear Element beyond the assumed start/stop extent, and then import it to native to clip it. Would you not be able to edit the start/stop stations within the Element Information for this under Side Slope Parameters? I might be missing more of the workflow. Please give me a little more detail, or a screen shot of what you are trying to accomplish. It sounds like you are requesting an enhancement, and I would like to document it correctly.
Holly