If you already have a .vdb file, RenderMan will read it with blobbydso:impl_openvdb procedural without converting the .vdb file to volume. Before Houdini 16.0.655, the only way is to create a box SOP and manually specify the Post include to inject it to the RIB.
Using Post Include
Make sure $RMANTREE/lib/plugins is in your Procedural Path. This is required for finding the impl_openvdb.so.
Create a box OBJ. In the box's Render/Geometry's Obj post-include, include the RiVolume that will be emitted to the RIB.
- Set your bounding box: the numbers in  after "blobbydso:impl_openvdb" is the bounding box for the vdb.
- Set the path and filename of the vdb after "constant string blobbydso:stringargs".
- Set the channel name(s). The default channel of your vdb file is not required to be "density". You just need to rename "density" to the channel in your vdb file.
- If you have more than one channel name, you can append it to the list, e.g. "constant string blobbydso:stringargs" ["$HIP/vdb/smoke.vdb" "density" "velocity"].