I have some liquids I bought in the shop. Now I would like more flavour, would I be able to just purchase concentrate and add it ? And will it need to steep as well?
You can, but you then run the risk of diluting the nic content at the same time, unless you are only adding a few drops of concetrate.
I would say you should let it it steep, if you are not getting a lot of flavour, leave it in a dark cupboard or drawer (away from kids and animals) with the lid off.
Some juices get better with time.
Maybe someone more knowledgeable will be along with any further tips and ideas?
I would agree with Doofry be patient and give it a bit of time for them to steep, another two to three weeks would not be uncommon. Adding flavouring would be a last result if all else fails imo. What liquid flavours do you have and what make?