You can select a large portion of the nodes in node management and select edit. If they all have something in common this will be easy to filter. If there is nothing in common with these nodes then you could create a custom property to assign temporarily to the nodes you want to change to snmp. Then group by the custom property and make the change en mass.
I'm not sure of a way to easily do this via SQL or SDK/SWQL at this time. But I'm slowly learning!