I was wondering if it is possible to use double pointed needles as straight needles for flat knitting if you put an end cap on it? I am asking because I plan on buying a set of interchangeable round knitting needles and a set of DPN. However, I found that the interchangeable needles are on the small side and I want to be able to have longer needles as well for larger projects and thicker wool since thicker wool= thicker stitches= less stitches on the needle. I can’t buy a separate pair of longer needles because it is too much money for me. Thanks! 
Well, you can, but end caps may not stay on the points well and you can’t have very many stitches. If its just a narrow scarf it may be okay.
What do you mean he circular needles are on the small side?
I meant that the needles are about 4 1/2 inches and if I were to use them as straight needles when taking off the cord they would be too small for me. In contrast the DPN are 9 inches. Or would 4 1/2 inches be enough even for thicker wool? I am still a bit new to knitting 
What Jan said, and what needles are you planning on buying? If you want longer needles, you just use a longer cable. You can use the circulars for knitting flat.
Knitting Flat With Circular Needles
Cables are available for the ones I use to make a 60 in. circular.
I plan on buying http://www.amazon.com/Knitters-Pride-Dreamz-Interchangeable-Knitting/dp/B006H7QXMO/ref=cm_cr_pr_product_top . I looked up the size and someone said they were 4 1/2 inches and another reviewer said they wanted to purchase these because they wanted to have needles that were smaller. The DPN are these http://www.amazon.com/Carbonized-Patina-Double-Knitting-Needles/dp/B000IG7TQY/ref=sr_1_2?ie=UTF8&qid=1374707535&sr=8-2&keywords=double+point+knitting+needles . Also, can you please tell me how to use circulars for knitting flat? 
Woops! I just saw you sent me a video on how to do it.
Oh, are you iknitwithcatfur?
GG post a link for how to knit flat with circulars. I never use straight needles anymore and don’t even own any. I use circulars for most everything.
OH so all I have to do it slide the stitches down to the cable as if it were a longer needle, then continue knitting as if with straight needles?
Thanks Jan in CA, at the time of posting my message I hadn’t reloaded the page yet and hadn’t noticed the video. Oops!
No, I’m not knitwithcatfur, but I do like her videos! I have the Knitter’s Pride Nova needles and love them! I also have the Sunstruck Options from Knit Picks. I love them too! You don’t need straight needles for knitting flat in addition to your interchangeables. In fact, I now find straight needles awkward and hard to use. I rarely use DPNs myself, small diameter knitting is a cinch with the circulars. I think that since you’ve decided to get the set, don’t order any others until you see what using them is like. Check the return policy with the site you plan to order from. Do they pay the shipping if you have to return something?
I like her videos too! Also, I read from previous reviewers that the company replaces things free of charge.