As long as you have warm relations and accomplish the rest of your goals that should be possible. I've been able to get the KPD to cooperate with me for President (both me supporting Thalmann and them supporting Braun) but never as part of my coalition.
Closest I've gotten was once I asked them to abstain on a vote of no confidence and they demanded to be part of the coalition instead. My remaining partners refused to consider it, not sure if it's possible rn. Maybe if the united left coalition is >50% and that event fires?
Maybe you can suck up to both the KDP and the Weimar parties enough to get them all to agree with it but that's a tall order.