Since most mono weed guards are tied from the fly head or from the fly head to the hook bend, it is far from impossible to add them AFTER the fact to many already tied bass flies.
I've done it myself on occasion with store bought bass flies.
It would also be a way to "slide" into into fly tying because all you need is some heavy mono, fly tying thread and some clear nail polish or head cement if you are very dexterous, or at minimum a cheap fly tying vise or a pair of Vise Grips and a fly tying bobbin.
Just a thought...
I will suggest that you create a flat spot at the very end of the mono with a pair of pliers or hemostats and get a few tight wraps behind the flat spot to keep it from pulling out of any thread wraps.