combining new with old LiPo cells?

[A RC Dude]

i had the 5th cell in my e-maxx to puff up becouse my balencer somehow turned off wile charging one time and it slightly overcharged. i can buy 2 new cells that are the exact same (minus the 25-30 chrages on them) for $40. would it be ok if i were to buy 2 new cells and wire them to the 4 i already have making it a 6s pack? or can you not combine new and old LiPo cells for some reason?