Is there anything that can convert my 1/8 collet to a 1/4?
You cannot go small to big but you can go big to small with collet insert from Inventibles.
What router or spindle do you have?
I have 2 collets for my router. I have a 1/8 and a 1/4. I intend to buy a 6mm as well since I have a 6mm shaft bit and the 1/4 won’t hold it tight.
I recommend not using adapters. The individual collets are generally worth the modest cost.