The use of the word "stealing" imply to deprive it's original owner of something. I think that incorporating some fragment of DNA material from one organism to another do not impact in any way the donor.
I start by clarifying this idea with an example. I will then give my opinion as to why borrowing some fragment of genetic code to incorporate in an adult is unlikely to give any visible result.
An example where that word would qualify could be: If I publish a book in which I copy a few pages from a popular author, such as Shakespeare. If my book appeal more than the original, if these few pages add value to my work and if these few pages are sufficient to convince some people to not buy the original source, depriving the original author of revenue that she would otherwise get, then this is clearly stealing.
This example is purposely exaggerated. An author can be considered as dishonest even if the source of the few pages was not from a well known author. A law suit in such case would help the original author to be discovered.
To answer to the important part of the question, I think that incorporating fragment of DNA from other organism is unlikely to help in most cases. For example, if a mammal would dream to get wings like a bird, such change can not happen on an already fully grown animal. A mammals would probably prefer to use the genes from another mammals, such as the bats, instead of using those of birds. Because the closest common ancestor between birds and mammals is much farther in time (possibly more than 300 millions year), the portions of the genetic code which are similar enough to be compatible and work in a meaningful way are very small.
The specific genes needed to change, for a large scale modification such as replacing the arms and legs with a wing system similar to bats, would need to affect the growth rate of early cells, soon after they start to differentiate.
The way the cells carefully orchestrate the rate of division, migration and pre-programmed cell death to create a viable miniature copy of an adult is an incredible amount of finely tuned apparently useless portion of DNA which actually may act as timers and other part of the genetic code controlling specific chemical triggers which act as equivalent to a decision making entity when described as an algorithm.
To give an comprehensible example, suppose you would try to build a machine which reproduce the symphony of Beethoven using thousands of coo-coo clocks, each one preprogranned to hit a bell at the right time. Let's say that you could group some of these mechanical clocks in clusters which would play the part of the music that repeat. A master clock would trigger each groups, onne by one, manage to rewind the clocks that already ding one time, to allow them to be ready to replay that portion of the song later.
Basically, a large part of the genetic code which appear to be useless for an adult is needed to build this complex 3 dimensional organism made of trillions of cells, like all pluri-cellular living organism do.
Once this structure is built, it can not morph to another one. Take the example of caterpillar. The metamorphosis to become a butterfly is done by demolishing the house and building a brand new one. Every organ is dissolved and a brand new set is developed from scratch. It is like if these two phases in the lives were two distinct creature encoded in the DNA. Half of the genetic sequence knows how to develop from egg to caterpillar.
Then, all the food accumulated by the caterpillar as fat reserve is used, similar to the yellow and white of a chicken egg, to restart the growth of a single cell inside the cocoon. That cell in the dying caterpillar restart developing like a freshly fertilized egg do, starting with a few divisions, probably 5, creating 32 identical copies. Then, starting to differentiate, each cell getting an almost identical copy of all the DNA, except small parts that account for cell specialization.
The only cell that get a fully intact DNA sequence, the entire instruction to start the next generation, are the sperm and egg. Every other cell get a copy with tiny difference, some parts of the code acting as lock to prevent the neuron cell to create the same protein as the liver cell, for example. *Bad example as I learned last week that the cells in the liver have mode than one nucleus).