Suppose I add external hardware to the Fx bin of the mixbus, is that summing?

Not really.
 
A summing mixer needs a feed from each track or bus and then sums it in the analog domain.
 
There is nothing inherently wrong with running your entire mix through analog hardware in an attempt to gain some analog warmth, but that's not summing anything.

As Bristol sez above.
 
Summers became a thing since many pro mixers received songs with many tracks or stems and had nice hardware at home.  But not a big Neve or SSL but they still wanted to use their neve EQ or LA2A etc. So to integrate the hardware they got simple line mixers to run the tracks/stems through and used the hardware they were used to using.  They didn't need a $100,000 mixer with every feature on every channel, but they don't want any cheap components in line either.  So clean, simple Summing mixers came into being, no compromise line mixer with maybe some monitoring features.
 
Another thing was Protools lower mixing bit rate.  Early PT hardware was limited to 24 bit internal mixing, I believe, when SONAR used 32 bit (internal) mixing.  My TC unit claimed 48 bits.  And when PT moved to 32 bits, SONAR was using 64 bit processing.  (my timeline may be off but the thrust is correct).  Some people found that mixing out into a board from PT got them a less strained sound than using the low mixing bit rates to mix in PT internally.  Since so many people use(d) PT it made economic sense to come up w/ external Summing units, even if you didn't have any fancy pants external hardware to use with it.  In that belief system, the more summing tracks the merrier (and less summing in PT), where 16 tracks and more were required.
 
What you are talking about is "mixing" a stereo out into external, analog hardware.  That would change the sound of your stereo track, so you would need to send it back into SONAR to record the new tone stamped on your track.  I do exactly that when mastering (note: I don't mix using the hardware).  But many people do mix through hardware (or software) on the two-buss - just be sure to capture the post fx sound.  
 

Thanks. I’m also curious about the differences between Digital Summing in a DAW, Printing a Track, Bus or Mix vs. Bouncing vs. Exporting. Are these just different terms for the same thing? Is ‘Print’ a function exclusive to Pro Tools ?

Reading info on the web and and watching some videos, not all are very clear.

You "render" in SONY ;-)  They should be all the same thing, tho I'm not a PT expert.  But there is no digital summing going on to "print" a track or bounce a softsynth (track).  Exporting a buss does involve "mixing" since (usually) you have more than one track getting shoved down a buss.

I built a balanced resistive summer to send bus stems to in a 1U box with xformer coupled makeup amp.
Worked nicely, and I liked the sound but fast bounce export would obviously not include it.
So I bounced the output of the summer back to a mixdown stereo pair in Sonar.
 
After while I missed the fast bounce and my old workflow so I ditched it and now just rely on the ProChannel's Console Emulator to provide the non-linear goodness.

I probably should have posted this in the Techniques Forum.  Anyway, here's a video I recently watched that got me thinking about summing mixers.  https://youtu.be/P_B0jCCdbUI
 
There was also this thread in the Software Forum: http://forum.cakewalk.com...periment-m3603902.aspx

Funny I thought the Mix 2 had more depth of field on headphones. 
We are always talking high end gear when we talk using analog summing. The shortfall is many who don't understand that fact think your going to improve your mixes by running them through a cheap analog mixer, sorry, not going to be the same as running it through the gear he used, Antelope convertors and Dangerous summing. 
And as was demonstrated, is the differance only very few can actually hear worth the price?