Linking Domain names

I own this site cherokeeschild dot com and my SEO says I need to add tee shirts to it so can I buy cherokesschildteeshirts dot com and link the two so they go to the same site

That’s a statement. Did you have a question?

yes sorry I need to know CAN I buy another site name cherokeeschildteeshirts dot com and it go to the same site as cherokeeschild dot com and keep both or do I have to start all over if I change the name which I really dont want to do

Yes you can. You need to setup 301 redirect trough htaccess file.
That will save all your previous SEO efforts.

What you need to do is park those domain in the same hosting .
When you build your backlink, remember that target only one domain rather than two to get faster result.

I would suggest that you keep both and host them

It really depends if you need to have two domain names. Why is the SEO suggesting you to buy that other domain, is it to have that for seo use or just because it has the “teeshirt” word on it?

Seems to me that having another is totally unnecessary in your case. Just saying.