I know that this may sound like a death wish, but I have an old harness without a belay loop. Can I tie my own belay loop?
Sure, and make your own seat belts while you've got the spool of webbing out.
Harnesses without belay loops are built that way for a reason: simplicity. What will you gain by adding a belay/rap loop -- and a homemade one at that! -- to your harness? If you add a loop, you add a complication and a potential failure point: Your tied loop will surely come untied as soon as you lean back over the Great Roof.
Stop dwelling on what you see as a half-empty cup and focus on what you do have: a beating heart. Instead of jury-rigging a suicide bomb, part with some cash and get a harness that has all the features you need. Yes!