Pappy,
Of course you have to give up your seniority number. They aren't keen on hiring airline guys in the first place, do you think they'd want to hire you if you won't resign from your current employer? They are dumping many thousands of dollars getting your type rating and training done, why would they let you keep your seniority number? Just so you can get called back off furlough from whatever airline you are at, and leave them with the bill? I don't think so. It just doesn't make any sense.