When is the deadline for the application?
If there is some time, you should create more concept art pieces and add them to your portfolio. If not, then just apply and don't worry. If you get it it's a "YAY!" If you don't get it it's a "continue doing/improving your art, keep your job (if you have one) and keep trying"
Hope your style fits the company style.
Also think about it in this way: if you have little experience in concept art and they were going to hire you and get you to start work on a good project this coming Monday (for wathever feature animation/movie or AAA game they may be producing), would you feel like you are able to do the job?
If you have little time to add some concept art to your portfolio, do it thinking about this situation. Because in the industry, you may be given 5hrs to finish a full piece for your team to add to the product or to give to some client. And if that is the potential case for this company, your portfolio should show them you can do it!
