@xmready, I've used a 12-word seed as an extension in the past.  When I was a younger bitcoiner I thought that would double my entropy, but I've since learned that it does not.  Take a look at hosseinimr93 post above, he is showing you how to generate an honest-to-goodness 24-word seed with double the entropy of a standard 12-word seed.  I also advise against using the same pool of words (i.e. Bip39 word list) for your extension, just to add an extra level of security.