Getting parked domains indexed?

[Input]

Is it possible to get parked domains indexed into the search engines?
I'll explain more below...
I have a client who owns a domain that features products from a couple of her companies. She then bought two other domain names (which were her two company names) and parked them on the main domain.
She's been giving out these domain names to her clients however her clients seem to put the domain names into the Google search instead of typing it in the address bar. Why they do this, I don't know.
Because the parked domains have no content, they aren't indexed in the search results and aren't being found by her clients.
Is there anyway to get those parked domains into the index or do they have to have their own websites with their own content? Can you have one page on those parked domains or is that considered bad SEO? (I think they call them doorway pages).
Thanks for your input!