If I bought a phone from somewhere else would US Cellular put it on my plan?

[Ash13]

I'm thinking about getting an HTC Merge from amazon or eBay or something because for one thing because US Cellular's are expensive, and for another thing I'd gotten an LG Triton from there and it's not working properly, we've tried switching the battery and they looked at it and said there was nothing wrong with it and claimed it to be the charger, but I didn't have the exact one that came with it and said they couldn't do anything till they had it, but I didn't know where it was. Now the warranty is up and the phone is getting worse and worse and I know it's not the charger. We've asked numerous times and they keep changing the date as to when I can get a new one, first September then November, and now December. I can't wait that long to get a new one, it has too many problems. If I bought a new one from somewhere else so I didn't have to pay $400 for the retail price on the new one on top of the new contract would they put it on my plan or no? Please! This is stressing me out and it's getting worse! I hate it!