If those items with SSI and MR on them have room for other mods to be added to them, then yeah keep them. If you want to add SSI or MR to other armor, all you want is items with alot of high intensity mods to unravel for "Relic Fragments".
The Imbuing system doesn't work like taking SSI off of one item and putting it on another item.
To add SSI onto an item you need the appropriate number of Relic Fragments, Tourmalines and possibly Luminescent Fungi.
To add MR onto an item, you need the appropriate number of Relic Fragments, Sapphires and possibly Ercu Citrines.