First, people are born trans, they don't "become" trans.
Second, transition is a long, expensive process that takes years to complete.
Third, the semen wouldn't survive that long in a syringe, it would have to be put in a sperm bank for storage.
Last, all that effort would be a waste because we don't have the technology to give trans people fully functioning reproductive organs. At least not yet; maybe in 20 years.