Adding shielding to existing cables

Um, no, my friend.  No...

Just build them with the right construction to begin with. Use Belden balanced wires with foil shield and inexpensive RCA jacks first, see if that fixes your problem. 

I was thinking of running a pigtail from the shielding and attachIng an eyelet to ground it to my preamp since there is a grounding lug. Would this work?

Yes and it is just the kind of low-risk proof of concept test I would try before cutting anything.

or is there a better way given what I intend to use?

Its hard to say what will be better without seeing exactly. Even then... it appears logical to assume shielding the whole length is what’s working. But then sometimes it turns out its the last millimeter of unshielded wire the noise comes in. Sometimes it even turns out its not the shielding at all. The shielded cable is quiet, the non-shielded isn’t, its the obvious conclusion. Only later it turns out it had nothing to do with that, it was just a poor connection at the RCA. Or a bad solder connection within the RCA. (I have had both.) So these things are not always as easy as they seem.

Which is why its best to go the low-risk route. Maybe you get lucky and solve it. Maybe you don’t. Either way you learn something you never even would have had the chance to learn had you jumped straight to cut and solder.

@erik_squires Why “no”?

from what I am able to understand, a shield is typically connected at one end (source end) via the rca metal body or chassis.  Doesn’t the rca plug ground connect to cassis ground?  If that is the case, how is running a ground wire from the shielding to chassis any different?

Lemming,

My "no" was to the extreme level of visually unappealing hackery here.

I mean, sure, add a shield and see where grounding it works best for you if that’s what you want to do.

From a "looks nice" and "doesn’t look like a mad scientists creation" point of view though, if you are going to start cutting off ends, why not just use some nice looking, shielded interconnect cables with new plugs? That may be all you need to stop the noise, and it will look nice and clean.

In other words, you are overthinking the solution already. Start with the normally suggested approach: Use foil shielded interconnects. You can make them yourself inexpensively.

THEN and only then if you still can't get relief start experimenting with novel grounding.

Oh, pro tip:

Building your own cables, use balanced cabled + a shield.

The inner conductors form the center and outer jacket connections.  The foil shield is connected only at the source jacket.