Proposition: There is no way, given some integer r and a sequence of iid random integer samples X1,X2,... uniformly distributed in [0,n) for some n>1, to always construct some finite k and function f : [0,n)^k -> [0,r) such that f(X1,...,Xk) is uniformly distributed over [0,r).
Proof: Suppose such a k and f exist. The distribution of U[0,n)^k is isomorphic to that of U[0,n^k) (just treat it like writing down a k-digit number in base n). And so f must be a function from a set of n^k things to a set of r things. By the pigeonhole principle there must be some integers x,y in [0,r) such that the preimage of x has size at least ceiling(n^k/ r) and the preimage of y has size at most floor(n^k/ r). By the fundamental theorem of arithmetic there exists (for any n,k) some r such that n^k/r is not an integer and so the probabilities of x,y (being proportional to the sizes of their fibres under f) are not equal.
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The gist of this is that you always might need to loop (repeatedly sample) for some ranges and you might need to repeat arbitrarily many times.
One way is by rejection.
Another nice thing is if you want to generate a Bernoulli r.v. for some probability p a computable number, you lazily compute the bits of p simultaneously to a random sequence of bits (distributed as Bernoulli(1/2)) and compare the two. If your random sequence is definitely less than p then generate 0. If definitely greater then generate 1. If not sure then generate some more bits.
In this way any Bernoulli random variable may be generated from an infinite sequence of iid Bernoulli(1/2), and basically any probability space can be modelled in this way too. In this sense, all of probability can be built out of tosses of a fair coin)