There is no direct yes or no answer to your question,the answer to your question will depend soley on what type root hormone you are talking about,being that different brand rooting products are made with different hormones or chemicals there is no blanket yes it will work or no it wont work answer.
You've got Acetic acids,then you have Butyric acids,Auxins & Gibberlins,all used for cloning,these are the only true hormones in cloning products,then you have Willow Extract as well as Red Clover,Dandilion root & way too many other natural active ingredients used for root propogation to list,again to all depends on what active ingredient the cloning powder/gel contains.
The only true plant hormones used for cloning are of the Auxin/gibberlin group,most cloning gels do not contain any real growth hormones,if you stick to cloning products where the active ingredient is part of the Auxin group of plant hormones then most likely there will be a benifit to added root growth from direct feeding to an established root system.
Adding Auxin Hormones directly to the soil or mixing them in the soil would not be the prefered or accepted delievery method,you'd need to mix them with water far in advance of using them in order for them to disolve properly,then feed the plants the hormone along with their watering for it to be effective.
Look for cloning products that contain Indole-3 Butyric Acid or Indole-3 Acetic Acid,these are the 2 most important & benificial plant hormones known to the science of botany,Shultz makes 2 different products,one is called "Take Root" & the other is "Root Tone",both of these powder based products have been shown in testing to promote extended root growth when fed directly to mature plants within their daily waterings,ive personally tested both these products durring different trials in my gardens & got great success using Indole-3 Butyric acid laced "Take Root" powder, i fed Indole-3 Butyric Acid in mass doses to select test plants in their waterings & grew some of the most jaw dropping buds ive ever grown,the bud in my avatar (The moose) is a real bud grown with Indole-3 Butyric Acid being fed directly to the plant through it's root system.
Here's some links i used heavily for reference in my experiments with different plant hormones using various applications,there is much usefull information within these links that directly applies to growing better fatter buds faster, but.................. its not a quicky subject nor are there any quick answers but its all here.
http://www.biologie.uni-hamburg.de/b-online/e31/31.htm
http://www.ext.vt.edu/pubs/greenhouse/430-102/430-102.html
http://web.archive.org/web/20060505053137/www.umanitoba.ca/afs/plant_science/COURSES/39-768/lectures.html
This is a great link here,it shows examples of vitiman B use in botany & puts to rest the use of the snake oil called "Super Thrive" that many people use to grow weed with & claim all kinds of great results,take the time to read all the links & follow all the extended links,it will take you several days research but when you finish you'll have a fairly decent understanding of what plant hormones are & what other natural products that are mistakenly refered to as hormones when used in cloning are,you'll also have a understanding of how to implement the Auxin's in different ways to promote growth in different stages of plant development.
http://www.encyclopedia.com/doc/1G1-12252342.html
http://www.weedscience.org/Case/Reference.asp?ReferenceID=717