I'm getting married soon and my cake is going to be 14in. 10in. and 6in. (stacked)... I really wanted to use my great grandmothers crystal pedestal cake stand... But its 9ins. in diameter... Is this possible? If so what would I need to do?

There are a couple of ways you could incorporate the cake stand.

If it isn't too tall, you can line the cake base with non skid fabric and set the cake on the stand.

If the stand is tall, say over 4 inches, you could use it to display candy or mints or some other favor. you could also use it to display flowers that match your cake.

A 9" cake stand was not designed to withstand the weight of anything larger than an 8" cake. I think the idea to use it for mints or something is a great idea. Unless you wanted to have a mini of your cake displayed on it.

You might be able to use it as a seperation between the 14 & 10 tiers.
Yes the 10" will probably be on a 12" board but it still should look nice.
If you decide to do this be sure to put lots of support doweling in there as a crystal stand will be heavy.