Are there US companies that will convert a used van?

I'm sure there is a company that will do anything you want.

Getting them to do it for less money is where you are likely to run into issues. A custom job is likely to have more hassles for them compared to just buying a new stripped down van and converting it similar to the last dozen that have rolled off the line. Odds are they will charge a premium for that hassle and it will likely more than make up for any savings you think you might get by buying the van and taking it to them.