How to automatically complete these edge loops?

Question

I'm still learning blender and I'd like to know if there's a faster way of creating these edges (marked in red) in my solid, continuing on the bottom face and up the backside, completing the loop, than manually using the knife to apply the cuts.

Answer 1

One of the method may be using a Knife Project tool. In Edit Mode select all the cross edges with Shift+Alt+RMB (with the edge select mode enabled). Duplicate them with Shift+D, then separate them with P-->Selection. Then, being in Object Mode rotate them along Y axis and scale them by Z axis. Now select the edges, then select the object and enter its Edit Mode (make sure you're in the side ortho view). Press Spacebar and type knife project. In a Tool Shelf (T) check the Cut Trough checkbox. Finally delete the edge loops with X.

Second method may be extruding the top part of the object and scaling it by 0. Select one of the bottom vetex in Edit Mode and snap the cursor to it (Shift+S-->Cursor to Selected). Change the pivot point type to 3D Cursor. Enable snapping tool (click magnet icon and set its type to Vertex). Select all the top vertices of the object, press E, Z (twice) and snap them to cursor. Next press S,Z,0. Finally remove doubles (select the whole mesh with A, then press W-->Remove Doubles) and recalculate normals (Ctrl+N).

Note: I don't know what your topology looks like, but if it looks like the one pictured below you may use a Grid Fill tool.

Answer 2

You can delete the vertices of the base.

Extrude the remaining vertices on the negative Z axis.

And scale the extruded vertices to 0 on the Z axis (S + Z + 0)

For a more detailed explanation read: how to snap several vertices to the same z.position