Do you have the expansion packs installed for Vanilla RCT2? If they show up in your vanilla game, then OpenRCT2 should work the same way. If you've just installed the expansion packs via their respective executable setups, OpenRCT2 should reflect that change the next time you open it. 

To see if the game acknowledges your installations you can simply see in the new game scenario window if any and all expansion pack scenarios show up in their respective tab.

If you want RCT 1 scenarios too Scroll down to Mr. Mans post after clicking this link https://openrct2.org/forums/topic/683-open-rct2-community-scenarios-packs/