Well, I went back at it and figured it out, and it's not as simple as the Hatsan Optima MP-A (semi-auto) manual makes it out to be so to save you aggravation here're the steps in detail.
1) unscrew the fore-end cap and remove.
2) remove the fore-end.
3) remove the plastic cylindrical insert from the top-inside of the fore-end. If you don't there's no way in hell the fore-end will fit back over the mag-extender. This part is tricky and three hands would help but you can do it - there's a metal split-ring preventing the plastic insert from being removed from the top of the fore-end. You need to carefully "walk" the ring downward until it slips off the cylinder. At that point the insert is removable from the fore-end.
4) remove the barrel to make this step easier. Using needle-nose pliers squeeze the magazine spring retainer (it's that black cap with two small holes inside the top-end of the mag). It's pressure-fitted so with your other hand ensure the cap and spring do not fly off. Wear safety goggles just in case. At this point you should have three pieces - the fore-end cap, the fore-end insert, and the mag spring retainer. Put them in a plastic bag for storage as you won't need them anymore.
5) reassemble the firearm minus the magazine spring. What you should end up with visually is the original configuration minus the fore-end cap.
6) insert the magazine spring and carefully compress it back into the tube. Press it all the way in and hold it in place with something flat and sturdy - I used a Home Depot paint stir-stick (your finger alone won't do because when you remove your finger to 'capture' the spring in the mag-extender the spring will inevitably fight you to bend and not go straight into the extender - try it and you'll know what I mean).
7) position the mag extender over the mag tub and carefully pull away the stir-stick or whatever it is you're using to hold the spring in place. The spring will leap right into the extender and at this point push the extender downward to engage the mag threads and screw it in place. You're done. The mag extender now acts like the fore-end cap you removed in step 1 which is why you no longer need it. Personally I think this design is pretty poor because going forward removing the extender to release the fore-end will also release the spring which previously with the mag-retainer was not the case. anyway, there you have it, enjoy!