I was told that wheelchair van conversions for used vans such as the Toyota Sienna & the Honda Odyssey must be performed by Toyota & Honda and that Honda no longer agrees to convert used Honda vans into wheelchair vans ?

I know there are certain features such as the power sliding door on the Chrysler Town & Country Touring & the Toyota Sienna LE models that are required for the purpose of wheelchair van conversions but do all used vans have to be sent back to the manufacturer for a wheelchair modification or can some repair shops that specialize in wheelchair van repairs install a wheelchair ramp/lift conversion of a van ?