Usually not. The rule is that you shouldn't. Occasionally, we'll use it in speech by mistake, especially if the sentence is long and confusing.
It would sound strange to say
"Because the food smelled good, so I ate it."
But, in speech, I might say something like
"Because the food smelled good, and I had been waiting for a long time before Bob told me he wouldn't come after all, and it was too late to go somewhere else, so i just ate it." Technically, this isn't gramattcally correct, but the "so" will remind the listener of the connection between the first half of the sentence and the second half.
However, the elipsis (three dots: ...) at the beginning of your example make me think something else might be happening here. Look at the sentence
"I asked him to buy it because he was rich, so he could afford it."
Here, the "because" is connecting "I asked him to buy it" and "he was rich", and the "so" is connecting "he was rich" and "he could afford it." If there are three clauses, then two conjunctions (连词) are REQUIRED. You usually shouldn't use "because" and "so" to connect TWO clauses, but you HAVE to use both of them if you want to connect THREE clauses.