You can index them manually if you want to, just as you can index system files and hidden files manually. It isn't recommended though, for different reasons. I'll state the reasons first, and if you don't care, you can skip to the end to get the "instructions" on how to do it manually.
Remote volumes: contents on these can change without your knowledge or intervention. Other users can change the contents of files, and add or delete files at anytime, and you might not have control over that. To keep the index up to date would require Spotlight to constantly monitor the remote volume, giving you a severe performance degradation on your system.
System and hidden files: System files change a lot. Logs, preferences, caches etc change all the time. To index them every time they change could lead to big performance hits. If you want to index the hidden files, pick the ones that don't change much, e.g. the man pages. Avoid directories and hierarchies that contain preferences, logs and caches that change all the time.
How to force indexing: start Terminal and use the "mdutil" command. Try "man mdutil" and you'll get a short list of options to use with the command. That'll take care of the remote volumes, and this was the easy part.
A more tiresome approach has to be taken when it comes to the hidden files and system files. For that I refer to a hint on MacOSXHints:
http://www.macosxhints.com/article.php?story=2005050222125145&query=spotlight
Finally, my advice on this: don't. Not unless the remote volume is on your own server at home, where noone but you add, change or delete files.