Keeping fluids in stock work the same way as keeping items in stock.

Create a Fluid Interface with a Crafting Upgrade in it. Then, specify the fluid that you want to keep in stock by clicking a fluid container (like a bucket) in the "out" slot. You can change the amount by left or right clicking on the fluid. The fluids will now be autocrafted if needed.

To make the stocked fluids available to your storage network, create an External Storage and put it on the Fluid Interface. Put the External Storage in fluid mode.

Make sure the External Storage is in "extract-only" mode. That way, Refined Storage won’t attempt to insert fluids in the Fluid Interface that is meant for keeping in stock.