Adding Elder influence to an item?

I want to turn a ring that's "Redeemer" influence right now, and add "Elder"

I haven't played for a couple leagues and things are very different now. I remember in the past you could do it through harvest crafting but not sure anymore.

There is a harvest craft that allows you to randomize influence. 1000 primal lifeforce per try, gl.

if the item is already Redeemer, and I do that, can it add Elder and become both Redeemer and Elder?

And if so, any idea on the odds of that?

No. If you want 2 influences, you need to go find an Elder ring, then use an Awakener's Orb on the two. It will keep one Redeemer mod and one Elder mod, then reroll the rest of the item. Note that the item you click SECOND is the base you will keep.