Are you sure that the external jack is wired in series? That would be unusual, and would complicate your plan of adding an external mid/tweet box. There are no off-the-shelf passive crossovers designed for series driver configuration, as far as I know.
If it turns out that the external jack is in parallel, then you do have the option of adding an external passive crossover in a suitably configured mid/tweet box. You'd only be able to filter the response of the mid and tweet; the woofer would be running full range.
This can be done with an off-the-shelf passive crossover; Eminence makes such. You might have to modify it a bit if the unused lowpass filter section, which would have been for the woofer, has a capacitor in a parallel leg - you want to cut that connection, as otherwise it can result in an unsafe impedance drop.
If your goal is to add mids and highs, you'll be successful. If you goal is to end up with a high-end cab on a budget, well that's a long shot without good measurements and a crossover tailored to the specific drivers you're using.