Hello There,
Thanks for writing in! Regretfully this isn’t a feature offered by X. It could be possible with custom development, but this would be outside the scope of support we can offer. You may wish to consult a developer to assist you with this. X is quite extensible with child themes, so there are plenty of possibilities.
Meanwhile, I think there are users who already have successfully have a fixed sidebar in their installation. You can reach out to them in the Facebook Usergroup: https://www.facebook.com/groups/xthemeusers/?ref=br_rs
Thanks for understanding. Take care!