I could not find any verified references as to which of the two possibilities listed below is correct, but based off the fact that he is non-existent parts can interact with the things around him [relevant AAM question] I am leaning towards the first.
Brook just acts like most people when reproducing and via the Yomi Yomi no Mi fruits powers, somehow it all just works. The sound waves in his chest bounce off something, his urine is aimed by something so it seems probable that if he just went through the motions, (whether or not his wife could feel anything) then his non-existent organs would also operate normally.
This is to cover all bases, and explain why it would be possible, although admittedly not practical if he was unable to become aroused or physically respond down there. Because Brook has other liquids, it is fair to say that he does have sperm and we know that if nothing is happening, eventually Brook would have a wet dream. I would like to refer to this article about frog underpants that was placed on the frog in a very early experiment to learn about the basics of reproduction where they placed specially made underpants on the male frog, placed him with a female, waited for them to mate and then collected the frogs sperm and analysed it. Well in theory, brook could wear a similar apparatus to collect and preserve his own sperm, which could later be used by his wife to produce a child.
Plus there is always adoption