Can I give a creature trample after blockers have been assigned?
Yes, you can. If you look through the comprehensive rules on the details of the combat phase, you'll find that after blocks are declared, the active player (in this case the attacker) gets priority:


  509.4. Fourth, the active player gets priority. (See rule 116, “Timing and Priority.”)


You can activate your trample ability now. If you activate it doesn't mean you rewind to the phase where you redeclare blockers (509.1, "First, the defending player declares blockers ..."). You simply move on to the next phase of combat damage.

If opponent died because he didn't declare enough blockers, then he's been outplayed.