Between "noon and noon" during the nomination period, yes -- but not after bidding has started (this is what you mean, right).
In other words, between noon today and tomorrow is the nomination period. If anything happens to open a spot up within that time, you are free to nominate a player.
After noon tomorrow, when bidding starts, no new nominations should be entered for that round. If you make a trade that opens up a spot at 10PM on Thursday, you can't just throw out a new nomination that round, though you may bid on players currently nominated.
My opinion on this matter is from a reading of the rules. In the League rules, deadlines are clearly defined without exception. To me, anything other than this would require a rule change.