Adding the link to the file will add the chm to your assets folder (in a chm subfolder) automatically. Each time you generate your chm you'd need to copy your new chm over that asset file before generating the HTML5 output.
To add the chm as a hyperlink you'll need to do a bit of a workaround as chm isn't one of the acceptable file types in the Open dialog box. Once you browse to the correct folder, delete anything in the File Name field and type *.chm then press Enter. Now you should be able to see the chm file to select.
Finally, if your help is located on a web server on the internet or intranet, rather than locally on your users computers, they may run into security issues when they try to open the chm, as Windows automatically blocks chm files downloaded from the internet, as a security risk. They would need to go in to the file properties to unblock the file. You can see information about a bunch of security blocks here (note the links to the MS knowledge base articles are unlikely to work, but you might be able to find them using the wayback machine if you desperately need further information): http://kb.helpwaregroup.com/ms-html-help/hh_info#HH14f
AdChoices