Training an alert bark is absolutely possible.
You will however fprobably find that training this sort of bark on your own won't work. While it's possible for an owner to train what is essentially a "speak" command that involves strong, consistent barking rather than just a single bark or two, the dog won't direct it at who you want it directed at in the circumstances you'd want to use it in real life because the dog's understanding of the exercise and motivation/mental state surrounding a "speak" won't transfer to that situation. You'd need to get the help of a trainer, one familiar with protection training and things like the hold and bark, to teach it properly for the use you have in mind.
The local SchH/IPO club is a good place to start looking for such a trainer. Most clubs keep their membership limited to those who want to fully participate in SchH/IPO, but they might be willing to entertain a part time membership for the purpose of teaching just an alert bark, or there may be a trainer/helper at the club willing to work with you outside of club to accomplish it.