Well, by the rules yes, you can extend it, since it has a duration of "1 full round". An extended version would thus run for 2 full rounds, and you'd have to say that all the effects of the spell would last for the duration.
Related question: at what point during the spell's duration does the spell cause damage?
1) To everyone immediately, and immediately to anyone who enters the AoE.
2) Continuous damage throughout the 1 full round.
3) To each person on her action.
This came up in last night's game, and I ruled #3, but am thinking that maybe #1 is correct. #2 gets confusing: how do you handle the damage of someone who enters the AoE immediately before the spell ends? Do they take as much damage as someone who was in the AoE for the full duration?
The disadvantage to #1 is that it doesn't make spellcasting difficult; I'd think I'd have a hard time casting a spell if ice were pelting me.