Can one hire an illustrator to make drawings for a paper?
It is entirely reasonable to hire a graphics designer or similar artist to make illustrations for scientific manuscripts, and I do in fact know people who have done so. For example, many journals now invite one to submit a "striking image" to be the splash graphic for the paper online, and this is a great place to get an artist involved.

An artist working on graphics for a paper would typically be given credit in acknowledgements but not be a co-author, since they have not made a contribution to the scientific work. There might be cases where "graphic design" might expand to become "visualization and analysis of data" worthy of an authorship, but those would likely be rare and relatively easy to distinguish.

The primary complication that this can add is one of copyright, as some journals require you to note images that need to have their copyright handled separately from the body as a whole. If you make an arrangement where the artist transfers the copyright to you, however, you can likely transfer it onwards to the journal without problem.