I don't know of any spelling programs that won't allow you to customize your dictionary with words you use repeatedly but that are for whatever reason not included in the program to start with. When you first fire up that program, make a point of teaching it words it should know. Teach it your name, your city's name, your email address, the name of your company, any names of people you contact on a regular basis, and any acronyms or specific terms you use in your writing. The current research says that they are, but I'm not quite convinced of it. At the very least, however, computer technology has changed the face of writing forever.
Most students are content to plug their writing into the grammar and spelling check and let it go.
Writers who rely on a computer to tell them what is wrong with their work will never learn to correct bad habits in their prose. Just like a kid who never has to pay for anything herself will never learn the true value of material possessions, a writer who never has to actually learn to spell a word will never understand the word and how to use it without their trusty computer. There is no way that the computer can learn the context of the sentence and how I meant it to be used. All it knows is that Their and There are, indeed, words. The use of a spell-checking program is something that needs to be done with caution.