We know that both the Star Kingdom and Manticore Ascendant sub-series occur in the same century Post Diaspora. The (currently) final book for the SK series ends in 1522 PD while the most recent MA book ends in 1544 PD. That's 22 T-years between Treecat Wars and A Call to Vengeance. Stephanie would be 37 T-years old at that point. So there's a real chance she could believably appear in the Manticore Ascendant series, but what do you think would be the best way for it to happen?
Personally I could see Travis and Lisa Donnelly crossing her and Climbs Quickly's path while on assignment or vacation to Sphinx. Maybe with Karl and Survivor appearing too. RFC seemed to hint in Best Laid Plans that Stephanie and Karl eventually married, but the passing reference didn't exactly have a lot of detail to it. I wouldn't mind knowing if they did marry, why did all of their descendants continue on with the Harrington name instead of the Zivonik name? And what happened to the title for the Barony of Zivonik since Karl's father was mentioned to be a Baron in either The Stray or one of the SK books? Was one of Karl's siblings older than him and therefore inherited the title? In addition, a Travis-Lisa meeting with Stephanie-Karl could serve as an opportunity to introduce the first post-Stephanie generation of Harringtons.
With Stephanie being 15ish in Treecat Wars, the 22 T-year time gap between that book and the time of A Call to Vengeance and the forthcoming A Call to Insurrection could mean her and Karl's oldest child could be around 12 T-years old (about the same age as when she was adopted by Climbs Quickly).
Anyway those are my thoughts for today. Maybe someone else will have an opinion too.