Is an Islamic prenup possible after marriage or is done or something while in marriage?

So yes, instead of getting a prenuptial agreement, which means signing prior to the marriage. You can get something called a postnuptial agreement.

Which is basically the same thing, but you're signing it after the marriage.

People sign postnuptial agreements like 10, 20, 30 years later.

You can sign an agreement between you and your spouse at any time before or during the marriage. Even upon divorce, the two of you can agree on how things are getting divided. The problem is that upon divorce there's so many emotions, so much background that people aren't able to come to a conclusion.

That's why it's so important to talk about it while you're still in love.