Ethereal Unique (repair w Ral + chipped gems)

No. Ethereal items cannot be repaired in any way - whether via blacksmith or cube recipe. They have be indestructible or self-repairing/replenishing if you want to use them yourself. You could put a

Zod rune in it, if the item allows sockets.

Supposedly there used to be a trick back in LoD (pre-D2R) with personalizing (

Anya's Act 5 quest reward) ethereal items, which would reset durability to max. If that ever did work (I never tried it myself back then), it has been patched out long ago.

Note: ethereal weapons are fine to use as a caster (who don't use the weapon itself to attack). Just make sure you don't have an attack skill on left click so you never accidentally swing it at an enemy. Weapons only lose durability if you actually swing it at an enemy via attack or skill. If you don't have a spell to set on left click can just put the Throw "skill" on it.