CJ
I would consider using the benefit you have from your decision of using your NPRM rather than putting a pole in the middle of your concrete pad.
Instead, you need to either modify your NPRM to accept an additional 3" mast as the end, with proper support struts (beefy angular iron), or make a new one.
Principle idea can be seen from this Baird Mount, which is for a 3" pole
If modifying your existing NPRM, leave the main pole in place, just in case you want to go back.
In this way, the load of the ballast stays in place weightwise, whereas the pole placement moves without the problems of the 90 degree arm extension as discussed by the other guys above.
The NPRM can be built from L-iron and a 3" pole, the trickiest part is where the pole attaches to the trays. But if you have welding equipment available, you should be ok with some supporting plate.
Note that the rest of the trays of my Baird is just bolted together, just as I would imagine yours is.
If you're really into the modular stuff, weld a 5" tube section to the end, and make a holder like the one
@RimaNTSS suggests.