How to add a link to an external site within admin?

I would like to create a link in the admin control panel to access the Plogger photo gallery admin I am using within Zencart. I want to add this link in a way that any future upgrades would leave this link intact.

For now I have just edited the admin/includes/header_navigation.php file to include the link to the plogger admin.

This does work rather nicely now, as I've edited plogger so that I'm taken to plogger's admin already logged in. Added a tab in plogger's admin to link back to the Zencart admin, it works great.

I just need a way to do this so the Zencart admin link to the Plogger admin will be intact after a Zencart upgrade.

Zencart version 1.5, clean install.

GreenGecko:

I would like to create a link in the admin control panel
...
For now I have just edited the admin/includes/header_navigation.php file to include the link to the plogger admin.

That's the best way. And, since it's just a few characters, it's something you can very quickly and easily transfer over when you upgrade next. That's the beautiful thing about tools like WinMerge.

Do pay careful attention to this point too:
ANY TIME you click a link in your admin which takes you TO ANOTHER SITE, you disclose your admin foldername to that other site. It'll show up in their traffic logs, analytics details, and any whos-online or similar type of tracking they might do.
If you don't want it disclosed, then DON'T PUT THOSE KINDS OF LINKS IN YOUR ADMIN.

Thank you DrByte for the information and for the warning about any admin links to other sites.

However that is not the case with how I have plogger setup. My Plogger setup is in the root of the Zencart site, on the same domain name not a different site.

So how much work would it be to port this plogger setup to an installable add-on for Zencart?

Is there somewhere that gives developers guidelines on developing add-ons for Zencart?