I'm considering adding a solar thermal space heating system to my house
which already has a gas furnace for hot water / baseboard heat.
Can the systems be integrated such that the solar system is the primary
source with the existing system secondary?
Certainly - if you're able to arrange your life around the times when
solar is available, or store enough of it. I feed the output of my
solar thermal panels into the same exchanger loop and storage tank as
my gas and wood boilers. Since I work at home on my own schedule, I
get up when the house gets warm, and work until it cools off again. If
I expected a warm house at 6 AM, I'd need hugely more storage...
You do need some intelligent device that knows the time and date and
your schedule to control it all. Controls based just on temperature
would typically run the gas heat at night and leave the storage hot at
the start of the solar day.
You'll probably end up designing and building such a system yourself,
at least I haven't seen ready to go packages.