How to isolate turntable from footstep shake or vibration

As others have stated, adding springs under a table having a spring suspension is a very bad idea (imagine a car having two sets of springs for each wheel!)

They’re called helper springs. https://www.tarett.com/items/996-997-991-products/996-997-991-suspension/996-997-991-shocks~springs/...

One way to consider going is to install Townshend Seismic Pods under not the base of the Oracle, but in place of the stock springs, with the Pods resting on the shelf, not the Oracle’s base.

Good idea. Which is why I said:

The most cost-effective answer will be if the Oracle feet can be replaced with Townshend Pods.

At AXPONA I visited Butcher Block Acoustics. Very nice turntable platforms. But I may be incorrect here, but I am not certain there is a substantial difference between them and one of the thickest maple cutting boards you can find on eBay for a lot less. So I put my VPI Prime Signature on top of one of those thick maple cutting boards. Never an issue. Done. 

   My turntable is neither shelf wall mounted or resting on a bedrock type of base. My table is an older one with no built in springs or other dampening devices. My house is over 100 years old, second floor, and moves slightly with season changes. I use multiple subs. Never have experienced a degradation in sound with these conditions but wondered if better isolation from vibrations would produce any improvement and, if so, would I notice it?
   In one of my previous posts I asked what could be done to isolate the table from unwanted vibrations. Thanks to millercarbon for his input. He recommended springs under the table. So, I ordered some Nobsound springs (since they're not very expensive). Even though I don't have any romping kids stomping on the floor I did notice a slight improvement in sound. MC also recommended Townshend pods which do the job even better but given their expense I decided to, at least, try the Nobsound first.
   So many factors in vibration control. Good luck in your quest.

You got that one right, there are an awful lot of factors that go into it, which is why I lean so heavily on educating on principles first and recommending specific solutions second. If I was there in person it would be pretty easy to eyeball and come up with a solution. Over email we have to fall back on generalities.  

Mass will solve most problems, if you can get enough of it. That's the trick. Fortunately mass is dirt cheap. Literally: one or two 50 lb bags of play sand, in a box several inches deep, put a piece of MDF or butcher block on top, you are pretty much golden. Put some Pods on top of that, turntable on Pods, now you definitely are golden.

You are doing it just like I did, Nobsound first as proof of concept, which then either is deemed "good enough" and you are done, or you like it and want more in which case Pods. If you do go to Pods remember Nobsound will work under all your other components, and you can make extra footers from the leftover springs. One of the all time great bargain tweaks.

"The most cost effective answer will be if the Oracle feet can be replaced with Townshend Pods." I disagree---that still amounts to adding springs to a turntable with a spring suspension.

And that is why I above suggested using a set of the Townshend Seismic Pods, not under the Oracle’s base (the non-floating frame which provides the table’s structure), but in place of the table’s stock suspension springs. Remove the Oracle’s springs, and install a set of three or four Pods under the floating sub-chassis---the bottom of each Pod resting on the shelf the table sits on, the top of each Pod in direct contact with the underside of the acrylic floating sub-chassis. That will isolate the sub-chassis from not only the shelf the table sits on, but also the table’s structural frame and it’s dustcover, as well as it's motor. In that regard, the Pods will provide an improvement in the sound of your table in general.

The Townshend Seismic Pods may be used thusly with any number of spring suspension tables; I have done exactly that with my VPI HW-19 Mk.5 (a Mk.4 with it’s stock platter and main bearing replaced with that from a TNT-5).