I found that you can however the flavours are not concentrated enough, check the ingredients list on the back and avoid any that have oils in them.
if you have a cake decorating shop nearby then lorann flavourings might work.
concentrated flavours for eliquid are soo cheap anyway that it doesn't make much sense to use others but if your determind to give it ago then I would recommend you buy some glycerine (sainsburys have it on offer for 48p for a 38ml bottle) and a flavouring and make 0% nic and test it in an old atomizer