yes, it is quite easy to do. If you're moving iTunes make sure it is consolidated (an itunes advanced preference).
First, stop iTunes and iPhoto if they are running. Second, you must copy the "iTunes" folder and/or "Photos" folder over to the external. Thrid, hold the option key and then click on either itunes or iphoto. A window will pop up asking where the library is. Simple point it to the respective folders on the external drive. When you are satisfied that everything is working as expected, you can delete the original folders freeing up space.
They will be a bit slower in displaying artwork or images but everything will work as expected.
It is also possible to copy only the iTunes Media folder and change an iTunes setting in preferences to look for it in its new location. Doing it this way will keep the image cache on the main HDD which makes using iTunes a bit quicker.
If you look closely at my iTunes advance preferences, you can see that I keep my actual media on a different disk than my boot disk.
Here are some of my tips for deleting or archiving data off of your internal hard drive.
You'll need to free up lots of hard drive space.
At a minimum, at least, 15 to 20 GBs of space.
Perhaps even more.
Hard drive full or near full:
Have you emptied your iMac's Trash icon in the Dock?
If you use iPhoto, iPhoto has its own trash that needs to be emptied, also.
If you use Apple Mail app, Apple Mail also has its own trash area that needs to be emptied, too!
Other things you can do to gain space.
Delete any old or no longer needed emails and/or archive older emails you want to save.
Look through your Documents folder and delete any type of old useless type files like "Read Me" type files.
Again, archive or delete any old documents you no longer use or immediately need.
Uninstall apps that you no longer use. If the app has a dedicated uninstaller, use it to completely uninstall the app. If the app has no uninstaller, then just drag it to the OS X Trash icon and empty the Trash.
Also, if you save old downloaded .dmg application installer files, you can either archive and delete these or just delete the ones you think you'll never install, again.
Download an app called OnyX for your version of OS X.
When you install and launch it, let it do its thing initially, then go to the cleaning and maintenance tabs and run all of the processes in the tabs. Let OnyX clean out all web browser cache files, web browser histories, system cache files, delete old error log files.
Typically, iTunes and iPhoto libraries are the biggest users of HD space.
If you have any other large folders of personal data or projects, these should be thinned out, moved, also, to the external hard drive and then either archived to disc and/or deleted off your internal hard drive.
You may have to Purchase an external FireWire or Thunderbolt hard drive to move these files/data off of your internal drive to the external hard drive and deleted off of the internal hard drive.