I know this is strongly hypothetical and would need to alter some of the movie for it to be plausible, but say that events transpired such that Kiara and her love for Kovu, and perhaps Simba's desire to keep her and Kovu apart,, would lead to Kiara sneaking off in the night and actually joining the Outalanders (not saying she'd support the plot against her father, just that she'd ditch her own pride and join Zira's pride so that she could be with Kovu.)
Would Simba try and get her back or probably disown her in anger?
I can actually think of a scenario where Kiara might do such a thing too. Lion King 2 is slightly based off of Romeo and Juliet. If you recall, I believe Juliet married Romeo but was actually promised to Paris or something. She ended up having to fake her own death (so she wouldn't get disowned by her father for refusing to marry Paris and so she wouldn't have to tell her father she married a member of the enemy household.)
What if Kiara was betrothed to another lion (supposedly the betrothal law was axed by Simba when he became king, though it's never officially stated) and made her move to leave right before the wedding was to happen?
Granted, I'm not sure she would join the Outlanders if this happened (there is, as Kovu mentioned, the option of both of them ditching their respective prides and forming their own pride), but it's possible.