I want to modify a pronoun with adjectives like "rich he", "poor they" and "beautiful I". Can I do that?
For example, can I say "I saw rich him driving a supercar", "Poor you can't buy foods enough", "Smart they graduated from a famous university"?
A good question! No, you can't (normally) put an adjective before a pronoun. All of your examples sound incorrect and non-fluent.
But there is one way that you can correctly put an adjective in front of a pronoun: as an exclamation. For example, Poor me! or Lucky you! which is like a way of saying "How unlucky I am!" or "How lucky you are!"
There might be a possible use for it: if you use it as some kind of "alias" for different outcomes. It's hard to explain, so I'll just present an example:
You find a suitcase full of money. You can either take it and become rich, or take it to the police and stay poor. "Rich you" could buy expensive things and live comfortably. "Poor you" would have peace of mind because he did the right thing.
It's not orthodox, but it isn't wrong either (as far as I know).
(Wrong) I saw rich him driving a supercar.
If you have not referred to him before then you should use:
I saw a rich guy/person/friend/neighbour driving a supercar.
If you have referred to him before then you might consider:
I saw him driving a supercar. Such a rich guy!
The key is to identify what you want to focus on, and the order you wish to convey the information or your opinion.
(Wrong) Poor you can't buy foods enough.
"Foods enough" is incorrect and should be "enough food". Probably what you want is:
You are poor and can't buy enough food.
If you want to emphasize the relation between the cause and consequence, you can use:
You, being poor, can't buy enough food.
You, as a poor person, can't buy enough food.
But note that all three sound quite rude if you are really talking to a poor person. And in a generic sentence only the first is suitable:
If you are poor and can't buy enough food, you can collect food stamps from the government.
(Wrong) Smart they graduated from a famous university.
If you have previously referred to those smart people then use:
Those smart people graduated from a famous university.
If you are talking about them for the first time then use:
Some/Many smart people graduated from a famous university.
If you want to be implicitly link smartness to the fame of the university (possibly sarcastically) then use:
They, smart people, graduated from a famous university.
Possible alternative sentences that use correct English grammar:
I saw a rich guy; he was driving a supercar.
There was a guy I saw; he must be rich because he was driving a supercar.
You are so poor you can't buy enough food.
They were smart; they graduated from a famous university.
You can replace any semicolon in the example sentences above with a period if you are willing to use multiple sentences rather than just one.
As already noted, there are a few very limited circumstances in which you can modify a pronoun with an adjective.
One of the ways you can do this is in the exclamation, "Poor you!" But be aware that this means "you are unfortunate," not "you have little money."
You could stretch this even further and use "poor you" as the subject of the sentence, "Poor you can't buy enough food." But this is only suitable for informal writing or speech, and it is not easy to know when you can get away with using such an expression. As a general rule, I would advise you not to try, at least until you have a much better ear for the language. Moreover, the sentence does not mean what you thought it meant: its meaning is close to the more formal sentence, "You are unfortunate because you can't buy enough food."
The opposite of "poor you" is "lucky you," not "rich you."