As far as I can see, from your configuration, you have basically a legacy quad lnb, two of which, whose ports are cascaded with two SW21's to additionally receive the satellite position at 61.5. One receiver must directly connected to the legacy quad on the Dish500 and gets only 110 and 119, but not 61.5. This is the black cable coming out of the Dish500. It comes down directly to the left of the grey box directly to a receiver, without being connected to any of the two sw21s.
You can get the 110 and 119 satellite on one additional receiver by directly running a coax to the fourth outlet on the quad on the Dish500, with no additional modification.
To use more than four receivers, you must switch to all DishPro lnbs. You can use a DishPro quad and a DP34 to allow you to use six receivers. However, only four receivers that get the satellite position at 61.5, for which you would need to connect a DishPro single lnb attached to the upper dish. If you want six receivers to get all three satellite positions, you will need a DishPro twin, two DP34's and a DishPro single. This would provide you with 8 outlets that would get all three satellite positions. If you get six receivers, two could be dual tuner units like the 322, 522 or 721.