It is not uncommon to make some changes to articles while they are in press. However, there are a couple of things you should be aware of. First, you ought to contact the editor, to find out whether there is time to insert anything. After a certain point, the publication schedule will probably not permit any changes. After that point, the editor may be willing to delay your paper until a later issue, or they may not.
The second thing, which is more specific to mathematics, is that the editor may not want to make any change that involves the actual mathematics without sending the article back to the referee(s). Mathematics journals have good reason to be very picky that every symbol and argument that appears in a final publication has been subject to the proper peer review. Additional background or explanatory material will probably be fine, but any change to how you calculate or prove something may not be possible without additional refereeing.