Hi Cathy, I believe you can. As I have added keywords to mine after a few Senior Artists made some recommendations about the types of keywords to use. However, I am not 100% positive.  Lynn

Hi Cathy and Lynn,
 
Yes you can, BUT we very conscientious about it. Don't add keywords that might be misleading or not accurate.  Remember to keep in mind
 
What the card IS
 
not
 
What it COULD BE  

There are many search engines that will "crawl" your cards' details, especially the title and keyword fields, and use these then to allow your cards to be indexed for use in search results.
 
Within GCU, this process can take >24 hours.  Outside of GCU (i.e.: Google search), it can take months.    
 
Be aware that if you change things later, like your title, then some search engines "reset" and you may lose any benefit of having a stable card in the search index.
 
The first words and phrases in both the title and the keyword fields are more important than later words.  You basically want to use words and phrases that shoppers (like you) might generally use in search but also make sure you add applicable specific words so that shoppers who are looking for cards with certain characteristics (like GOLF for a Father's Day card, or RED ROSES for an Anniversary card) that your cards will surface.  
 
Good luck!
 
TOM