Is there anyone discovered new drugs by actively create mutations in drug-producing organism?
I'd say this is possible, but not really practical. Mutating genes by UV light is not that hard, it's just the screening for the (new) compounds that makes it difficult. You would probably be better of creating diversity using established chemical methods.

Usually UV mutagenesis is done for whole organisms, but some people probably also tried this for single genes (even though they'd hit the whole genome). This is the easy part; you could get an enormous amount of diversity with very little work. Even though mutations that change enzymatic products are probably much more rare than mutations that change substrate specificity or product chirality, people find those mutations (example from the top of my head: p450 hydroxylation patterns, although they probably used targeted mutagenesis). 

Determining if a specific compound is produced by an organism is also not very hard. You break all the cells, put the solution on a column and check with your favorite analytical method whether or not your compound is in there. Checking for new compounds is much harder. You need better analytical methods such as MS/MS as you don't have analytical standards, and the adapted molecule might not even be detectable with the analytical method you used for the starting molecule. We are very good at detecting needles in haystacks, but determining if there is a needle, a stick, or maybe a piece of hay from another species in the haystack is much harder. You'd already have to predict what kind of altered molecules you expect to find. 

When you have successfully done all this, you have identified a single new molecule. Nowadays, people use libraries with >100,000 compounds to screen against their disease protein of interest. 

The idea of mutating enzymes to produce new products is not that strange, people work on this, publish, find a lot of interesting things and we learn a lot about enzymes. As an approach for drug discovery this approach is just not high-throughput enough.