You can get 'dab converters' which plug inline with your aerial.  See
for example ebay item# 5860602002.  The ones I have seen are really
intended for use with home HiFi, but are probably adaptable to use in
the car, although it seems a bit messy to me.

Alternatively, I know that some companies are developing an inline
converter which passes on each dab station on it's own frequency, with
the DAB station info embedded in the RDS headers, so you will be able to
use your car radio to select the DAB stations just as if they are just
normal stations.  Don't know when these are likely to be out though.