Sorry, there currently isn't anyway to copy/paste individual choices into a multiple choice field. The choice list has to be typed in.(Note: you can copy/paste the entire choice field including choices so once you created your choice list you can use it in other places/forms)
Others have asked for this, you can vote for this idea here: http://forums.adobe.com/ideas/1484
We use this to help prioritize our work
How does one copy the entire choice field? I've tried that and it doesn't seem to work. Does it have to be copied into the same form?
Are you using Chrome? Someone else reported in the forum that copy/paste isn't working Chrome today. Try a different browser if you can and tell us if it works.
You just select the object hit CTRL-C and then CTRL-V to paste.
Wow, that was quick. Yes, I was using Chrome. I just tried it in Firefox and it worked fine.
I think a Chrome update changed something that is causing copy/paste to fail. We are invistigating...