I have managed to purchase an unopened factory sealed Apple Watch but it's older than one month.
Is there any way I could purchase AppleCare+ for the watch?
Call Apple you never know. My extended warranty on my Mac is over and they still give me free support when I call. I brought it up to them and they said we know, no problem...
That's not playing by the book if it's older than 60 days, but an Apple Store manager might be able to make an exception. OP has nothing to lose by asking.