Making my website accessible to the largest amount of people... starts with a simple problem

Hello,
I was trying to conceive a navigation bar of my own for some advanced effect. I really like to keep the content inside of it made in Foundry 3 though, as it will make maintenance much easier than explaining my CSS odyssey (to hell) to the one that will be in charge.

I do have a small problem/complain nonetheless. I can’t find a direct way of saying to a container that it should have a tabIndex and be targetable by the “tab navigation”. It’s a bit annoying considering it would be my buy something button and I like money. I cannot let these 3 tab users go without buying me something can I ?

Jokes aside, many disabled persons use the “tab navigation”, it would help them a lot if it was easy for us to set one up.
So (unless I am missing something and in that case I shall remain silent forever (wouldn’t hurt if you ask me)), would it be possible to add to the containers an option to set up the tabIndex ? Something like the “data-” option that we already have.
Thank you!