Saw some seasoned guys do this on the jobsite today:

They are running a rack of conduit on strut. After they filled the strut with all the conduit that could fit, the super tells them he forgot a pipe; need to add one. So they strapped a short section of strut to the bottom of the just-installed conduit (full rack); then with all-thread extending from this new short section of strut up above the full rack, they added another short section of strut, upon which they were able to install the "forgotten" conduit.

To summarize: the new pipe is supported by strut which is supported solely by pipe (which is itself supported by strut which is supported by purlings).
If you can visualize this, please tell me: doesn't this violate some code about not using wiring methods to support other wiring methods?

300.11(B)

There are a lot of other ways to solve this problem. For instance you could use one hole straps Tek screwed to the bottom of the the existing strut. You could use coupling nuts to add more all-thread to the existing all-thred and add anotherlayer of strut. You could remove the wires from two of the existing conduits and use those tubes as your structural supports. [Add a couple more conduits to re-route the wires to the lower level on this one.]
I am waiting for someone to come out with a UniStrut extention. It would basically be a piece of steel channel which would slide into the end of the existing strut and be bolted into place.
~Peter

Peter - I don't think any of your other solutions would have worked. See, actually there were already two layers of conduit installed (two unistruts back-to-back off the purlings, with one layer of conduit above and one layer below). The remainder of the existing all-thread was too short to get a rod coupling on to add strut below; besides, there are ceiling height restrictions. Thanks though.

Caddy clips ["Bat Wings"] [K-8 or -12] to the all-thread.
Does the inspector check the wire fill?
If you have ceiling clearance problems, then wouldn't the existing situation also cause this? Are they using shallow strut?
Why does the extra pipe have to be on strut anyways?
~Peter

If you have a nut on the rod, then that nut could be replaced with the rod coupler, which is just a longer nut.

I know it's not code compliant, but do you really think its unsafe.
It's been happening since Thomas Edison's day.
I see an lot of support violations especially above suspended ceilings.

The caddy BC beam clams will clamp to a 3/8" threaded rod. If you had to you could add more conduits this way. I know its not listed for rod, but it works in a pinch.

Caddy should make a clamp with a 1/4-20 or 3/8-16 hole so we could attach to the side of a 3/8 or 1/2 rod for conduit support and box support. Other than the bat wing style.
That would solve a lot of problems.

I came up with it first and call it the Massachusetts Tim. Email me so I can tell you where to send the royalty check to.

Peter:

They do make Unistrut Couplings (joiners) which would serve the same purpose.

You can see some here:
http://www.unistrut.com/Browse/cat_detail.php?S=S07&P=P2900

and see all of them here: http://www.unistrut.com/Browse/cat_pg.php?P=S07_Elect