Crafting a powerful robe is a service available at a clothiery.
The clothier will take one of your robes, and one of your amulets, and tailor the robe from them. This will free up an extra slot for an amulet. The robe is impregnable (erodeproof).
For example, if one brought a +3 black robe, and an amulet of life saving, to a clothiery, he could combine them into a +3 robe of life saving.
These items can be wished for by phrasing them, 3 blessed +3 ornate robes (of +3 magic regeneration, immutable)