are R packages open-source?

I'm trying to modify some of the commands in add-on packages, and I'm
wondering if they are open source, which would allow me to do that?  If so,
does anyone know how I could access the code?

Jeff has already mostly answered your question, but I'll provide what 
may be a bit of clarification.

All R core packages and R itself are released under the GNU Public 
License (GPL), and you are thus free to modify their code.  There are 
other restrictions on attribution and distribution in the GPL that are 
spelled out in the license but basically come down to crediting the 
source, noting your changes, and distributing your modifications always 
under the GPL as well.

The vast majority of CRAN packages for R are also distributed under the 
GPL, also allowing you to change the source.  As Jeff has pointed out, 
you need to check the License section for the package you're looking at 
either on CRAN or in the downloaded package itself.  Feel free to ask on 
this list if you're not clear on the licensing status of any particular 
license or package.

In something that might otherwise go unsaid: If you make changes to a 
package to either fix a bug or enhance the functionality, *please* give 
back those changes to the community that you have benefited so much from 
by sending them to the package maintainer at least, and 
possibly/probably also posting your modifications to this list if they 
are finance-related. Everyone benefits when we share our work and code 
on published analytical techniques, even though the professional 
investors among us will continue to compete in the markets.

Regards,

   - Brian