Youd be all good to sign up, though for whiteboards at heavyweight events (they have featherweight sideshows) you could potentially just turn up (best to tell them your coming of course) - only downside is pit space is only guaranteed for heavyweight teams.

Also whilst itd be ok to sign up if you had the robot now - just be aware there are other requirements before you enter the arena, like a tech check at most events. You can find those forms in the rules.