Some crafting recipes may require a filled fluid bucket. To use fluids from your fluid storage to fill buckets, you can set up a processing pattern that has following inputs:
1 empty bucket
1000 mB of the fluid of your choice
For the output:
1 filled bucket
Put that in a Crafter that faces a bucket filling machine (that for example Thermal Expansion has). Refined Storage will output the fluid from your fluid storage, and, an empty bucket. It is up to the external machine to fill it.