Hello Robbie!
thank you for your question!
Store manager for OpenCart allows to import special prices, discounts can be added in lower grid:
https://www.screencast.com/t/8cZrTrmyctP
Please also vote for this feature request here:
https://support.emagicone.com/878216-Import-of-discount-price
The more votes the feature gets, the sooner it can be taken into consideration for development.
Thank you!