I think you just have to use Buckets.
Easiest way though.. fill in the layer below the 'roof' of the reactor with cobblestone, completely.
Then, dig out ONE block of the cobblestone at a time and place a bucket of the liquid onto one of the sides of that new space, opposite to the side that's a source liquid block.
Repeat for each cobblestone.
This way, you get source liquid blocks everywhere which results in a more efficient reactor.
Automating large amounts of Gelid Cryotheum is fun though
-Wuffle!