I might be missing something, this seems very simple if you use a DAW. Do you use midi? I use ableton live, but there are free alternatives that work just as well. Trouble is you have to learn to work with the DAW. What a DAW does is take different inputs + virtual instruments, and gives you a workspace with layers upon layers to layer on additional inputs, effects, feedback loops etc.

In your case, I'd load up a DAW, load in the vsl/spitfire plugins as instruments on parallel tracks or in a rack, get them to output dryest signals possible, then on the master bus or at the end of the rack I'd load in my favorite free reverb plugin, find a fitting preset, and probably set it to somewhere around 5-30 % wetness. Now they're in the same space!

With a DAW you can program or play live(with midi keyboard), and you can set it up so pressing a single key will trigger a multitude of instruments in different plugins at the same time, and you can route all those outputs through a single channel reverb plugin. And then play it back later and play around with effects, edit midi notes etc.

I am a bedroom producer, not an engineer or composer, so I know very little about a lot of stuff. Hope it helps tho.

PS if you already have dry multitrack recording of your thing, you can use something as simple and free as audacity, since audacity can run plugins and layer audio tracks. If you want more power, or if you wanna have more control and flexibility you might wanna learn to use a DAW.

PPS Here is a small, free reverb plugin that I like https://www.u-he.com/cms/179-protoverb and here is a list of free DAWs, with videos. http://routenote.com/blog/the-10-best-free-daws-available/