Aesthetically, wearing a stole or cord with a hood seems unappealing. Practically, there is a slight chance it would complicate the hooding process. (It is surprising how hard even simple things like processing down an aisle can become when it's for a big life event, so making sure the hood goes on symmetrically and doesn't get caught on a stole or cord might be hard.)
Honestly, if I were you I would not consider wearing the stole and/or cord unless they have a deep cultural meaning to you (e.g. African-American stole of kente cloth). If they're purely academic or about undergrad activities--say, you did not wear an honor society stole for your baccalaureate graduation because you were embarrassed and now regret that--then don't look like you're bragging about your old undergraduate career while getting your Ph.D. You can take whatever pictures you want with the old stole and cord on your new gown, but focus on your current achievements at the ceremony.
If it is something deep--cultural recognition, perhaps recognizing a deceased parent--I describe next how to figure out the rules and a then a possible alternative.
Figuring out what the rules are and/or how to get permission.
- For the doctoral hooding, talking with your advisor and the department staff in charge of this may work best. Above all, if the person hooding you (likely your advisor) disapproves, then do not disrespect them by insisting on it. The department staff will probably also know the rules for convocation.
- Have you attended a prior graduation at your institution, and/or can
you find pictures? I did not attend my own convocation for my
doctorate, but many people (esp. undergrads) had decorated hats,
inflatable toys, signs, etc. A meaningful symbol would not have been out of place.
You could probably wear a small pin (especially enamel, like a lapel pin) on the front of the robe.
You may be able to decorate your hat. I haven't seen Ph.D. students do this, but many bachelor's and master's students do.
Have family/friends bring something to put on right afterwards, like a meaningful stole or a flower garland. (Flowers would NOT survive if put on before the hood, anyway.)
Wear the stole under your gown and open the gown later when you take pictures. You may even be able to set it up to poke out of your gown, as the collar of a dress shirt would.
Wear something meaningful that shows despite your gown. For the sake of example, kente cloth pants, traditional footwear, a flower behind your ear, etc.
Congratulations! You're almost there!